Mr. W. G. Horner on the Vascular Spectrum. 269 



tissue paper should be wafered upon one side of the board. 

 The box being set on an elevated stand at a distance from the 

 observer, and a light placed within it, the cardboard, with 

 the papered side towards the light and the slider horizontal, 

 must be fixed against the box, and turned in the direction of 

 the observer, who will then receive no light from that quarter, 

 except through the two little apertures. He will seat himself, 

 with due regard to firmness of attitude, close to a table in 

 front of him, and on which is a single light, below the level of 

 his eye, and at the distance of rather more than four times the 

 geometrical focus of the lens he designs to apply, and having 

 his unemployed eye comfortably cased and bandaged. 



With one hand he draws down the lower lid of the open eye, 

 so as to expose the sclerotica freely ; while with the other he 

 holds the lens at such a distance and position between the 

 light and his eye, that the focus shall fall upon that an<de of 

 the eye which is opposite to the aperture in the cardboard. 

 A mirror will materially assist in acquiring this adjustment at 

 first; but to a practised observer its success is announced by 

 the certain though dim appearance of the vascular shadows. 

 Having succeeded in placing the focus, and at the same time 

 keeping his sight fixed upon the aperture in the cardboard, 

 if none of the shadows are situated vertically below the aper- 

 ture, he will direct an assistant to remove the light slowly in 

 the requisite direction, until one of the shadows has arrived at 

 that position. Then carrying the lens sideways until the focus 

 arrives at the other angle of the eye, his sight being steadily 

 intent upon the aperture in the cardboard as before, but his 

 attention being also carefully directed to the progress of the 

 selected shadow, he will notice whether the latter vanishes 

 when vertically below the hole in the slider. If not, an assist- 

 ant must move the slider in the requisite direction until that 

 effect is obtained. It then only remains to measure the di- 

 stance of the observer's eye from the card and the mutual di- 

 stance of the apertures. 



As all the misleading causes tend to magnify the estimate, 

 it will be found that my original statement of 1 foot to 15 feet 

 is more than double of the actual quantity. The seat of vision 

 will therefore prove to be less than ^Lth of an inch behind the 

 system of vessels. This locus of accurate convergency will 

 probably be found to be the posterior membrane of the retina. 

 It seems, however, to admit of doubt, whether the distance 

 from the system of vessels is a permanent quantity. 



To account, under this altered statement of the distance, 

 for the phenomena of Art. 9., we must suppose that the 

 origin of the vessels, and consequently the larger vessels 



