﻿Mr. J. C. Douglas on Shadow Optometers. 343 



if not, the kind of correction required, both of which, without 

 the application of the shadow method, must be left to trial and 

 the opinion of the observer, which (particularly in the case of his 

 being inexperienced) must render the process tedious and the 

 result more or less inaccurate. 



As the methods described depend on the passing of a shadow 

 over the field, and are thereby distinguished from other methods 

 of determination in which the object is regarded directly, I have 

 called them shadow methods ; and I suggest the name Shadow 

 Optometer as appropriate to such instruments constructed on 

 these principles. The purposes to which the principles described 

 and explained are applicable, and the manner of applying them, 

 are evident; the special arrangements in any particular case 

 (upon which the accuracy of any method must in a measure de- 

 pend) will depend on the ingenuity of the experimenter. Pos- 

 session of a pen-nib and a set of lenses would enable any ordi- 

 narily intelligent person to find the kind of spectacles he re- 

 quired (if any) with great exactitude by viewing a distant street- 

 lamp in the manner indicated. 



After completing the above, I found the inversion of a small 

 object placed near a hole in a card held near the eye had been 

 described by M. le Cat in his Traite de Sens (Brewster's Edin- 

 burgh Journal of Science, vol. iv. p. 89) ; but his description 

 is incomplete as given in Brewster's Journal of Science, and 

 his explanation appears to me erroneous. It is stated the pin 

 and hole must be near the eye (ibid.), whereas any luminous 

 point may replace the hole in the card, and the phenomenon 

 depends on the distance of the card being less than the focus of 

 the eye having its conjugate on the retina at the time of experi- 

 ment. The explanation given is, that the shadow of the pin is 

 seen, and the inversion is due to the fact that the light from the 

 lower part of the white wall or window, which furnishes the light 

 in M. le Cat's experiment, passes by the upper part of the pin, 

 and that from the upper part passes by the lower part of the piu ; 

 hence the shadow is inverted with reference to the wall or win- 

 dow. I consider the correct explanation is that given by me, 

 and that inversion of the light from the wall or window with 

 regard to the pin has nothing to do with inversion of the sha- 

 dow, which is produced equally well if a luminous body replace 

 the hole in the card. I have not noticed that the shadow ap- 

 peared to be on the opposite side of the card to the eye; and the 

 diagram given in the notice referred to (ibid.) is evidently in- 

 correct; it removes any possibility of the explanation given being 

 that given by me. The enlargement of the shadow is due to the 

 same cause as would enlarge any shadow modified by the effects 

 of refraction, and not to any circumstances peculiar to this case, 

 as shown in the diagram referred to. 



Calcutta, Feb. 7, 1868. 



