RETINAL DISTURBANCE. 405 



tic thalami. In their most highly-marked state the former may "be 

 treated of as results of insanity of the retina, the latter as of cerebral 

 vision. 



Disturbance of the retina, brought on by any cause whatever, may give 

 rise to simple spectral apparitions, which, as the circumstances A aritions 

 change, will have an indefinite contour or a definite form ; nor from retinal 

 are they merely shades and shadows : they may be presented disturbance - 

 in colors, which, however, are usually dim or subdued. Thus, if, the eye- 

 lids being closed, we press gently with the tip of the finger on the inner 

 or outer angle of one of the eyes, a gray spot surrounded by colors makes 

 its appearance on the opposite side of the same eye, and dances about as 

 the pressure of the finger varies. With a more extensive and heavier 

 pressure clouds of various rainbow tints fill up all the imaginary space 

 before us. In like manner, the passage of an electric current from a vol- 

 taic pair induces a flash of light of considerable brilliancy. Internal 

 pressures and spontaneous variations in the rate of metamorphosis and 

 nutrition of the retina act in a manner analogous to external disturbances. 



From the muscge volitantes, which may be regarded as the first rudi- 

 ments of apparitions, it is but a step to the intercalation of simple or even 

 grotesque images among the real objects at which we are looking ; and, 

 indeed, this is the manner in which they always offer themselves, as rest- 

 ing or moving among the actually existing things. I do not undertake 

 to say how far we are liable to practice deception upon ourselves, after 

 the manner we have spoken of in children, when we have once detected 

 the fact that we are liable to this infirmity. An inanimate object for in- 

 stance, a stick is seen upon the floor; we go to take it up; we find there 

 is nothing there ; we return to our first position, but we can observe no 

 shadow or other reality that can be offered as an explanation of what we 

 have seen. An event of this kind predisposes us, perhaps, to return to 

 that disposition of exaggeration so natural to our early life, and the next 

 time the retina deceives us we involuntarily give to the hallucination mo- 

 tion and a more definite form. 



Insects flying in the air, or, rather, floating in vacancy before us, pre- 

 sent the incipient form of retinal malady. It may be provoked by un- 

 due use of the eyes, as reading by lamp-light. I remark it constantly, 

 in my own case, after prolonged use of the microscope. In a more ag- 

 gravated form, it less frequently occurs as producing stars or sparks of 

 light. From the earliest times, physicians have observed that it is a 

 "bad sign" when the patient localizes these images. " If the sick man 

 says there be little holes in the curtains, or black spots on his bed- 

 clothes, then is it plain that his end is at hand." 



Under the title of pseudoblepsis, or false vision, medical authors enu- 

 merate several varieties of the foregoing phenomena ; but when, as is 



