342 Mr. J. C. Donglas's Reply to Mr. Templeton's 



sufficient evidence, that the eye is simply a camera obscura, with 

 a lens to form a picture, a retinal surface to receive a picture, 

 and an adjusting-power for distance." That inaccuracies exist 

 in works on " physics " is not to be denied ; but text-books on 

 experimental physics are scarcely the sources from which to ob- 

 tain authoritative statements on difficult points in physiological 

 optics. The comprehensive work of Professor Helmholtz*, the 

 works of Donders, C. Stellwag, von Cariou, &c, would be truer 

 representatives of the opinions of those who are building up this 

 branch of knowledge. I do not find such physicists consider the 

 eye any more like a camera obscura than it really is. Helmholtz, 

 for instance, states in one place, the camera obscura is the optical 

 instrument most similar to the eye in giving real images of ob- 

 jects ; in another place, the eye acts on incident light essentially 

 as a camera obscura (Physiologische Optik) ; but he fully elu- 

 cidates the differences, and calls in the analogy where it assists 

 his description ; he discusses the various hypotheses of accom- 

 modation ; considers not the lens only, but also the other refrac- 

 tive media, the several curvatures, the great relative thickness of 

 the refractive media, &c. Mr. Templeton is no doubt aware 

 images may be viewed through the sclerotica both in the dead 

 and living eye, and in the latter by the ophthalmoscope ; while 

 accommodation is universally experienced, is observable with the 

 ophthalmoscope, producible in the dead eye by galvanism, and 

 measured in the living eye more or less accurately with the op- 

 tometer. In production and reception of the image and in ad- 

 justment is not the analogy proved ? 



Comparative study of the visual apparatus would be as efficient 

 in assisting to a correct knowledge of the eye in man as the 

 study of comparative physiology has been in the development 

 of human physiology; but observations to be useful must, I 

 submit, be accurate, and the publication of experiments which 

 do not even give any qualitative result, positive or negative, is 

 undesirable. Mr. Templeton states he has endeavoured to find 

 the principal focus of the crystalline lens without success by ex- 

 periments which " carry with them a suspicion of inexactness." 

 I submit such experiments are useless, as no conclusion can be 

 drawn from them : they do not afford even a useful degree of 

 probability, the matter they are instituted to decide remaining 

 as doubtful after as before their institution. The principal focus 

 of the crystalline lens has been measured by Helmholtz, a refer- 

 ence to whose experiments f will point out the precautions to be 

 taken, and may explain why Mr. Templeton's experiments were 

 not decisive. 



* Physiologische Optik. 

 t Op. cit. pp. 79, 80 & 81. 



