390 On IJghL 



less distinct J nor less v'widy than those of the prismatic 

 image. 



At times, the disease, which has been so often refer- 

 red to, is attended with peculiar affections, arising from 

 an unnatural accumulation of lymph, or some other fluid, 

 either above, or on the left side, of my left eye. Among 

 these affections are the following. 



At times, a bright spot, irregular hi its figure^ and like 

 the Sim in its lustre^ and hue^ appears in the field of vis- 

 ion ; brilliant even in the day time for a considerable pe- 

 riod, in each instance ; uniform in its aspect ; and lasting 

 from a quarter of an hour to two hours. In many in- 

 'stances these spots have, during a part of the process, 

 lost their lustre, and suddenly become black. The lus- 

 tre of the spots I attribute to an unnatural pressure on a 

 part of the optic nerve ; the blackness, to an increase of 

 that pressure, so great, as to interrupt for a season the 

 sensibility of the nerve in that place. 



When the disease in this form has reached the utmost 

 height, which I have experienced ; the field of vision has 

 been crowded •with lucid appearances^ sometimes fixed, 

 and sometimes moving. They have also been of different 

 forms. Those, which were fixed, have resembled a con- 

 fused collection of pieces of white glass, or rock crystal, 

 cut in the form of parallelopipedons, bevilled on both 

 sides, and at both ends ; and then held up to a sky light. 

 In several instances I have almost lost my sight, when 

 this affection has become intense ; and supposed myself 

 on the verge of an apoplectic fit. But, in every case, 

 vigorous exercise has in a short time dispersed these un- 

 pleasant symptoms. This effect I attribute to the mere 

 increase of the circulation. 



The facts, already mentioned, may be sufficient to 

 warrant the following conclusions. 



1st. All the appearances^ denoted by the word, visible, 

 are inherent in the nature of the Optic nerve ; and not in 

 the nature of light ; nor in the nature of the objects, by 

 which IT IS TRANSMITTED TO THE EYE. We are 

 prone, if I mistake not, to suppose, that there is a lustre 

 in the element of light ; that it is coloured with seven 

 distinct hues ; and that it diffuses over the face of nature 



