210 Mr. Harvey on a Case of Vision with regard to Colours. 



him, first on a white ground ; secondly, on a yellow ground ; and 

 lastly, on one of a green colour ; and in each case, he regarded 

 the two colours, to use his own words, " as well matched*." 



The day being favourable on which the last experiment was 

 performed, a vivid and well-formed solar spectrum was thrown 

 on the wall. He pronounced it to be composed of two co- 

 lours, yellow and light blue ; and which, in a former conver- 

 sation, he described as the ordinary appearance of the rain- 

 bow. The vivid and brilliant red of the spectrum he could 

 by no means distinguish. Its general appearance he regarded 

 as in some degree beautiful ; but the bursts of pleasure which 

 escaped from my children, as they contemplated the brilliant 

 colours in succession, appeared to excite in him the greatest 

 surprise. I afterwards found, that he considered the prism 

 as a thing moderately interesting, but as by no means meriting 

 the praises which had been bestowed on it. 



His eyes appeared to be excellently well formed, and time 

 has but slightly impaired their powers. According to the 

 opinion of my friend Mr. Tracey, surgeon, they appear to 

 possess all the essentials necessary to good sight; namely, per- 

 fect transparency of the several humours, a proper degree of 

 convexity of the cornea and ball of the eye, and to which may 

 be added, the perfectly healthy functions of its appendages ,• the 

 proofs of which are discovered in the just adaptation of the 

 eye to distance. In his communication to me on the subject, 

 Mr. Tracey observes, " If I might adduce any one point 

 (which under common circumstances, I should not be disposed 

 to notice), I should say the gray colour of the irides is much 

 fainter than usual ; and that the pupillse are extremely small." 

 The cause of the last-mentioned fact, is no doubt to be attri- 

 buted to the constant exercise of the eyes by candle-light, it 

 being known, that, persons similarly occupied, are of necessity 

 obliged to bring the object very near, and thus, from long 

 habit, produce artificially a permanent diminution in the mag- 

 nitude of the pupillae. 



In the present limited state of our information on this very 

 interesting and curious subject, and with so few cases that 

 have been hitherto presented to the philosopher, it may be 

 premature to offer any very decided observations on the cause. 

 The objects of a true and legitimate philosophy are perhaps 

 best fulfilled, by diligently collecting facts, and, by cautiously 

 deducing inferences from them, to form gradually the succes- 

 sive links of a chain, which shall ultimately lead to the true 

 cause. 



* A philosophical friend who was present, placed the colours under se- 

 veral different circumstances, but with the same uniformity of effect. 



Of 



