562 



SCIENCE. 



[N. S. Vol. II. No. 43. 



flections ; take, for instance, this idea (wliich is 

 developed by Chevreul in his voluminous 

 vohime On the Law of the Simultaneous Contrast 

 of Colors) that those colors which, when com- 

 bined, approach the nearest to white, produce, 

 when placed side by side, the most agreeable 

 effect. This opinion regarding the comple- 

 mentary colors is too absolute, as has been 

 sho-wn by a number of recent works ; but it is a 

 first step in the science of the harmony of 

 colors. 



The lesson ' On Vision ' terminating the work 

 is a brilliant resumi of the knowledge of that 

 time, together with the writer's personal ideas 

 on the subject of ocular dioptrics, showing from 

 the point of view of design in Nature all the ad- 

 vantages of the optical construction of the eye. 

 The eye, as we know, is a sort of camera obscura, 

 at the back of which is the retina. Before 

 reaching the retina the luminous ray must pass 

 through an ensemble of transparent media, which 

 offers all the advantages of the convex lens 

 without its inconveniences. We know, further- 

 more, that the luminous rays which strike the 

 margins of the lens have their focus or point of 

 meeting in advance of the focus formed by the 

 rays which penetrate the center of the lens, to 

 obviate which disturbing feature in photo- 

 graphic instruments a diaphragm is devised 

 which cuts off the marginal rays. The iris of 

 the eye, that membrane with its constantly con- 

 tracting and expanding circular opening, placed 

 in ft'ont of the crystalline, exercises the same 

 function. Young shows how Nature aids still 

 further in this direction by giving to the crystal- 

 line a gradually increased density from surface 

 to center, thanks to which the focus of oblique 

 rays is made almost to coincide on the entire 

 concave surface of the retina. 



The memoire ' On the Mechanism of the Eye ' 

 is the most important of the work ; it is remark- 

 able for the complexity of means employed. 

 The laws of refraction in media of uniform den- 

 sity had long been known ; the author shows 

 how the total index of refraction of a lens varies 

 according to the index of refraction at the sur- 

 face, when the density of the sphere or the lens 

 varies according to the power of the distance 

 between the surface and the center. This ap- 

 plies to the crystalline. He shows that the 



total index of the lens is greater than the index 

 of the center when the index, as in the case of 

 the crystalline, is greater for the center than for 

 the exterior. The number he got for the 

 total index was found almost precisely correct 

 by Mathiesen. It is. in this same memoire that 

 appears for the first time a description of the 

 Optometer with which Young made his principal 

 determinations, the most complete that have 

 ever been made of a single eye ; but the instru- 

 ment never came into general use and the trans- 

 lator was unable to find any trace of it. The 

 principle upon which it was constructed (by 

 Scheiner) is of a simplicity truly remarkable ; a 

 straight line down an oblong cardboard, and an 

 ordinary visiting card perforated at one end 

 with two pin holes, will give a fair idea of this 

 ingenious device. By closing one eye and plac- 

 ing the other at the pin holes, which are held at 

 one end of the straight line, the liue in question 

 appears no longer as a single line. To the eye 

 looking through the pin holes two distinct lines 

 present themselves which gradually tend one to- 

 ward the other till they meet, and their point 

 of meeting constitutes the nearest point of dis- 

 tinct ^dsion. Any one may possess this simple 

 contrivance home-made ; by a system of gradu- 

 ation on the cardboard one may readily obtain 

 a correct measurement of the position of the 

 eye's focus. The experiment will show how 

 difficult it is to fix the point where two lines 

 cro.ss, and the reason is that it requires con- 

 siderable practice to be able to place one's eye 

 in perfect repose, i. e., to paralyze its accommo- 

 dation. 



Young had a large, prominent eye ; which 

 last feature he utilized with a compass to obtain 

 an exact measurement of its length, which he 

 found to be 23.11 mm. It was with his optom- 

 eter that he discovered astigmatism, the un- 

 equal refi-acting power of the eye in different 

 meridians ; the exact degree of which he meas- 

 ured by the focal distance of the lens adapted 

 to its correction. This visual defect he here 

 speaks of for the first time, but without seem- 

 ing to attach to the discovery any special im- 

 portance. Young presents very complete no- 

 tions on the following subjects : the field of 

 vision ; the field of perfect vision ; the sensi- 

 bility of the retina, less at the fovea than at 



