In 1836, Schnaitman, a Philadelphia optician, invented a ground bifocal lens. His invention consisted of grinding a weaker curve on the upper half of a lens of stronger curvature. The great objection to these glasses, however, was their extreme prismatic effect, which proved so annoying to the wearer that they were soon laid aside.

The next improvement we learn of was made in 1865, by a Frenchman, Secretan, of Paris, a relative of the original maker of bifocals, whose invention consisted in cementing a segment having a very sharp edge and a somewhat semi-circular form, on the lower portion of the distance lens. This small supplemental segment enabled the wearer to read with the lower portion of his glasses, while the upper part was for distant vision. This was a most decided improvement on the preceding forms, presenting the advantages of both, while avoiding many of their inconveniences.

In 1866, a Boston optician named Gregg, secured a patent on a form of lens very similar to this French one; that these French glasses were satisfactory is proven by the fact that some of them are still in use.

Cylinders had not yet been employed in bifocal lenses to correct astigmatism; however, in 1888, August Morck secured a patent on a bifocal lens that combined a cylindrical and spherical correction. This glass consisted of a distance lens of the sphero-cylindrical conformation desired; the lower portion was ground out in a semi-circular form and a segment of the appropriate reading combination was inserted in the place of the section removed. The two glasses were so adapted that the line of separation was least perceptible and had the distinct advantage of being exactly centered; that is, the optical center of each lens coincided with the geometric center of the corresponding lens. This lens fell into disuse when the French bifocals were made with sphero-cylindrical combinations.

Bifocals then reached so high a standard that oculists and opticians thought they had a lens which would forever remain an example of perfection. They had overcome the prismatic effect of the Schnaitman lens, the line of junction of the Franklin, and the prismatic effect of the Gregg glass, and thought the patient would forever now be content, but they had reckoned

without two powerful hosts-vanity and sensitive nervous sys

tems.

In effect, the lens that had come to be so much admired by all was gradually looked upon, by the ladies especially, as not so near perfection as it was at first thought to be; that line of junction of the near segment with its knife edge was not so imperceptible after all; in fact, when one knew it was there and had seen it a few times, it became quite noticeable; but what sealed its fate and made it an eyesore was the remark of some people, that it made the wearer look old.

In 1897, Borsch, of Philadelphia, devised a glass, which was termed the depressed bifocal. To make this lens he took a piece of Crown glass of certain index and ground it according to the required distance correction; in the lower part of this glass a circular or semi-circular depression of 17 millimeters in diameter was made into which he inserted a lens of higher index of refraction according to the strength required for near. The lenses were cemented together and gave a bifocal, the surface of which was but slightly interrupted. These lenses were further improved in 1898, by the same inventor, by enclosing the small segment between two plates of glass forming the distance correction.

The latter are very neat in appearance and delicate in construction, and combine most of the desirable features of bifocals; they are, however, in three pieces, cemented together with Canada balsam which, under certain conditions, is liable to become loose and clouded; then again, these lenses are quite expensive, so that they are within the reach of a limited number of persons only.

For a number of years the author has been endeavoring to devise a lens which would embody all the desirable features of a bifocal, from the oculist's, the optician's and the wearer's point of view. These desirable features are as follows: a bifocal having no perceptible line of junction between the near and far portions of the lens, no prismatic displacement, no cement to become loose or cloudy, a lens having the optical and geometric centers for the near and distance correction corresponding. with each other, and, what is of no little importance, one that can be made by any lens grinder and at so modest a cost that it may be within the means of anyone requiring bifocals to gain a VOL. CLIX. No. 952.

21

livelihood; and lastly, a bifocal that won't make the wearer look old.

This has been successfully accomplished, and the lens is made in the following manner: A depression is made in a piece of ordinary optical glass of refractive index of 1.53, and into this depression is placed a glass of higher refractive index, varying according to the strength of the lens required. These two glasses are placed in a special muffle, through which a current of electricity is made to pass, which causes the two glasses to coalesce or melt together without altering the indices of refraction of the component substances. This new piece of glass is taken from the muffle and given to the grinder, who grinds the required focus according to the distance prescription. The difference in density of the two glasses used will give the required near focus without any further grinding.

show you the result of these several years of work by exhibiting to you the solid invisible bifocal lens.

Book Notices.

The Telescope. Optical principles involved in the construction of refracting and reflecting telescopes, with a new chapter on the evolution of the modern telescope and a bibliography to date. By Thomas Nolan, M.S., A.M. (2d Edition, revised and enlarged, 16 mo., pp. 128.) New York: D. Van Nostrand Co. 1904. (Price, 50 cents.)

This volume forms one of the well known "Van Nastrand Science Series." The author has brought his subject up-to-date by adding a new chanter describing briefly the advances made since 1880, when the first edition appeared, and including a useful bibilography. W.

American Compound Locomotives. A practical explanation of the construction, operation and care of the compound locomotives in use on American railroads. By Fred H. Colvin. (8 vo., pp. 143.) The DerryCollard Company, New York; The Locomotive Publishing Co. Ltd., London. (Price, $1.00.)

'The railway engineers and the locomotive builders will find Mr. Calvin's work an exceedingly useful compendium of information pertaining to the compound locomotive as developed for use on American railways. W.

American Meter Practice. By Lyman C. Reed. 8vo, pp. 196. New York: McGraw Publishing Co. (Price, $2.00.)

The railway engineers and the locomotive builders will find Mr. Colvin's output of central electric power stations, and the electrical engineer will find it most useful should he have occasion to need a selection of the commercial meters, or to compare them in respect of efficiency and accuracy.

W.

Franklin Institute.

[Proceedings of the Stated Meeting held Wednesday, March 15, 1905.] HALL OF THE FRANKLIN INSTITUTE,

PHILADELPHIA, March 15, 1905.

PRESIDENT JOHN BIRKINBINE in the chair.

Present eighty-four members and visitors. Additions to membership since last report, twelve.

Prof. Lewis M. Haupt read a communication on "Tunnel Engineering with especial reference to the Simplon Tunnel." The paper was a brief historical sketch of the advances made in this branch of engineering by the introduction of improved drilling tools and high explosives.

Mr. Byron E. Eldred, of New York, was introduced by the Chairman, and gave an informal account of his "Method of Regulating Temperatures."

Mr. H. I. English, representing the Gray Telantograph Co., gave an account of the evolution of the telautograph since its first introduction in 1893. These communications were freely discussed and were referred for publication.

The subjects of the remarks of Messrs. Eldred and English were referred to the Committee on Science and the Arts for investigation and report.

The Chairman extended the thanks of the meeting to the speakers of the evening and the session was adjourned.

Wм. H. WAHL,

Secretary.

Committee on Science and the Arts.

(Abstract of Proceedings of the Stated Meeting held Wednesday, March 1, 1905.) DR. E. GOLDSMITH in the chair.

The following reports were adopted:

(No. 2310.) Contributions to the Study of Documents, and Especially a Method of Quantitative Colorimetry. Dr. Persifor Frazer, Philadelphia.

ABSTRACT: The contributions of applicant to the scientific study of documents, the determination of the individual character of handwriting and the detection of fraud and forgery, including in particular the wedge-shaped color prisms devised by applicant for aiding in the work of examining inks constitue the subject of this investigation. These contributions are contained in a published treatise of Dr. Frazer on this subject termed, "Bibliotics, or the Study of Documents," which the members of the sub-committee have examined, and express themselves satisfied that it is a clear, comprehensive and masterly exposition of this difficult subject, and a work of high scientific and literary merit.

The Sub-committee has also examined and tested the device of the color prisms submitted by applicant. These consist of gently-tapering wedges of tinted glass, which are placed over the writing, in a strong light and moved across it from the thinnest to the thickest end. While so doing there will occur two critical points; the first, that at which the ink marks appear of maximum darkness relatively to the background against which they are

contrasted; the second is that at which the ink marks are no longer recognizable against the background; that is, they disappear.

The observation of one or both of these points, measuring carefully the distance from the point of the wedge at which they occur, gives data characteristic of the relative intensity of the ink mark tested.

In the opinion of the sub-committee, these prisms in the hands of one skilled in using them, are capable of giving approximate quantitative values to the relative color values of ink, and that the results of their use are of valuable assistance in the study and comparison of documents.

The sub-committee notes further that the mathematical discussion of the properties of these prisms, investigated by Prof. Geo. H. Hallett, of the University of Pennsylvania, shows that their selection and application are based on correct scientific principles, and this fact, therefore, increases the evidential value of the information obtained by their proper use in the hands of an experienced observer.

The investigators recommend the award of the John Scott Legacy Premium and Medal to applicant for his contributions to the science of bibliotics. (Sub-Committee, Joseph W. Richards, Chairman; Jos. H. Hart, Samuel P. Sadtler, C. C. Heyl.)

(No. 2331.) Electric Protective Devices. The Intern'l Burglar Immunity Co., Philadelphia.

ABSTRACT: The system of applicant is protected by letters-patent of the United States, numbered 725,900, 730,422 and 735,324, and dated respectively April 21, June 9, and August 4, 1903.

It is designed for the protection of bank vaults, safes, and isolated places in general, and depends essentially upon the use of several circuits and the employment of relays in these circuits, which, by their becoming active or inactive, as the case may be, bring the alarm system into action.

The main claims of superiority over the existing systems are based upon the use of a separate current, whereby the system derives part of its protective qualities from the place protected, instead of from a central station. The system is further protected by current derived from an outside source; for instance, an electric lighting circuit, the tampering with which at any time causes an alarm.

The report finds the system to be well designed, and well adapted for the purpose for which it is chiefly intended, namely, the protection of isolated places. A Certificate of Merit is awarded. (Sub-Committee, Richard Gilpin, Chairman; Geo. P. Scholl.)

(No. 2281.) The Art of Reducing the Attenuation of Electrical Waves and Apparatus. Michael O. Pupin, New York.

Previously reported, and made the subject of a protest from Mr. C. J. Reed. The Sub-Committee suggested some minor amendments in its report, in consideration of Mr. Reed's criticisms, and re-ffirmed its previous conclusions. As amended, the report was adopted. The Elliott Cresson Medal is awarded to applicant. (Sub-Committee, C. C. Heyl, Chairman; E. A. Scott.)

The following reports passed first reading:

(No. 2340.) Method of Color Photography. Michael and Henry M. Miley, Lexington, Va.

(No. 2352.) Phonosphere. The Phonosphere Co., New York.

(No. 2358.) Automatic Pistols. The Colt's Patent Firearms Mfg. Co., Hartford, Conn.

(No. 2360.) Method of Making Replicas of Diffraction Gratings. Frederic E. Ives, New York.

WM. H. WAHL,

Secretary.