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



falls upon it is not fixed, but has a motion contrary to that of 

 the shadoivs. 



Another observation, which may prove essential to further 

 researches, is this (4), that with sunlight admitted as above, 

 although the peculiar structure of the spot in question is al- 

 ways distinctly enough seen, yet it never appears so bright 

 as when the light falls on the verge of the cornea, in the re- 

 gion of the ciliar circle. On using a lens and candlelight, the 

 silky texture is scarcely perceived, unless when the focus falls 

 on the ciliar region of the eye. The shifting mass of light 

 which, at this time, appears on the spot, would seem to have 

 entered the crystalline lens laterally through the colourless 

 portions of the integuments, and to be thence transmitted by 

 reflection*. 



The easiest way to obtain correct measures of the quantities 

 of motion, would doubtless be by using a sunbeam fixed by 

 a heliostat in a darkened room ; but I have not the means at 

 hand. It requires some practice to manage a lens, and the 

 operation is at best complicated. Besides the apprenticeship 

 to the use of the lens, the method itself of observing, namely, 

 by indirect vision, is at first perplexing, and demands a con- 

 stant effort to resist the habitual tendency of the eye to follow 

 the object, and by that means exaggerate its apparent motion. 

 The choice also of external objects of comparison is limited 

 and nice : to fix the eye, they must be as few and as distinct 

 as possible, and be surrounded with entire obscurity, since 

 even a faint light in that situation would sensibly affect the 

 distinctness of the shadows. At the hazard, therefore, of be- 

 ing thought triflingly minute, yet to prevent vexation to other 

 observers and the unnecessary wear and tear of a delicate 

 and invaluable organ, I will describe the method which, after 

 numerous trials, has proved the most satisfactory in my own 

 case. 



A box, of adequate size, and capable of completely con- 

 fining the light, yet provided with proper apertures to admit 

 a supply of air, should have one of its sides substituted by a 

 large dark- tinted cardboard prepared in the following man- 

 ner. A sliding-piece of several inches in length being let into 

 it, and two holes punched, one on the axis of the slider, and 

 the other on its continuation upon the cardboard, a sheet of 



* Probably from the white folds of the corpus ciliare: this will account 

 both for the appearance described above (3), and for the variable lunula 

 observed by Mr. Wheatstone (Art. 7«). — By the by, in Purkinje's experi- 

 ment, are not the shadows entirely due to the light which permeates the 

 ciliar region ? 



