Mr.W. G. Horner on the Vascular Spectrum. 263 



The results of my own procedure in repeating that experi- 

 ment are too remarkable to be concealed, whether justice to 

 myself or to the cause of physiological science be considered ; 

 and as Sir David Brewster announces an immediate commu- 

 nication on the same subject, it appears to me that courtesy 

 to him requires me to offer mine through the same channel. 

 Besides, it is possible that our researches may have lain in 

 different directions; and even if they happen to have led to the 

 same termination, Sir David's prior claim will be established, 

 and yet the deductions of an independent, though less in- 

 formed, inquirer may have their value. 



1. C. W., like his predecessor Purkinje, employed a flame, 

 held within two or three inches of the eye, and on one side of 

 it, or below, to produce the spectrum. On trying this, and 

 assisting others to do the same, it presently occurred to me 

 that the image of the flame in the focus of a powerful lens 

 must be more convenient and effective. This substitution was 

 no sooner made than, on assisting another person to observe 

 the appearances in his eye, I discovered that the effect was 

 certain, only when the light fell on the ball of the eye, or the 

 sclerotica, without touching the cornea. 



2. It was likewise obvious to remark, that the lines in the 

 spectrum were never so dark and well-defined as when the 

 focal image was accurately formed. 



3. Placing the flame at a distance from my eye equal to 

 fpur times the distance of the geometrical focus from the lens, 

 and holding the latter halfway between the eye and the flame, 

 it is obvious that I obtained the maximum of illuminative 

 power ; and by applying such light to the exposed ball of the 

 eye in different directions, corresponding portions of the vas- 

 cular image were brought more distinctly under cognisance. 

 Thus a light near the right-hand angle of the eye displayed 

 the left-hand portion of the image, and so of the rest. The 

 central parts, within the range of distinct vision, were deve- 

 loped, in every instance, with splendid accuracy and minute- 

 ness. 



4. Towards the middle of a sheet of cardboard of a darkish 

 tint, having cut an aperture about half an inch long by one 

 tenth of an inch broad, I repeated the experiment by the di- 

 rect light of the sun, with still superior effect. 



5. In all experiments upon one eye, the comfort of the 

 other eye contributes materially to success. A case should 

 be bound over it, so as completely to darken it, without touch- 

 ing the eyelids. It has, by all observers, been experienced, 

 that a distinct view is not to be maintained, unless the light is 

 kept iik motion. The lens or the cardboard must be moved 



