with regard to Colours. 261 



been hitherto presented to the philosopher, it may be premature 

 to offer any very decided observations on the cause. The ob- 

 jects of a true and legitimate philosophy are perhaps best ful- 

 filled, by diligently collecting facts, and, by cautiously deducing 

 inferences from them, to form gradually the successive links of a 

 chain, which shall ultimately lead to the true cause. 



Of the hypotheses that have been proposed to account for the 

 phenomena in question, that which refers it to an insensibility of 

 the retina to certain impressions of light, appears at once the 

 most simple and philosophical. Some eyes, it is well known, are 

 capable of performing the general functions of vision, and are yet 

 unable to perceive those minute impressions of light, which to 

 other eyes are readily perceptible. A retina may be perfectly 

 adapted to receive the due and proper effect of a beam of light, 

 and yet, from some peculiarity in its organization, incapable of 

 perceiving all its component parts. In the present case, the ge- 

 neral objects of vision, as relate to form, distance, and magnitude, 

 were perfectly fulfilled, and the sensations arising from white, 

 yellow, grey, and the lighter varieties of blue, appeared in gene- 

 ral to be correct ; and it would therefore appear probable, that 

 the retina was sufficiently sensible to receive correct impressions 

 from pure white, up to colour of a certain intensity ; and, be- 

 yond which, ' s its power being enfeebled, it communicated only 

 imperfect and confused ideas *. 



In the cases published by Dr NICHOLLS, in the Medico- 

 Chirurgical Transactions, the anomaly appeared in some measure 

 hereditary ; in one instance being derived through the father, and 



* The point where this change appeared to take place, with respect to blue, was 

 between ultramarine and flax-flower blue, corresponding to 28. and 29- of SYME. 



