398 SUBJECTIVE IMAGES. 



The eye is adjusted to the varying intensities of light by the motions 

 of the iris, which admits more or fewer rays according to its 



Adjustment to . . > 



variations of state of contraction, an action which, on certain occasions, is 

 brightness. a i(j e( j by the orbicularis palpebrarum, which, "by bringing 

 the eyelids together, limits the number of rays passing to the pupil. In 

 man, the muscular fibres of the iris are of the unstriped form ; in birds 

 they are striped. Our perceptions of the intensities of light, as gather- 

 ed from the state of the iris, can never be so distinct as the indications 

 for sound yielded by the tensor tympani and stapedius muscles. In 

 birds, however, it is probably different. We gather, to a great extent, 

 our notion of the brilliancy of light from the rapidity of structural change 

 taking place in the retina itself. 



Although many images may be simultaneously existing upon the ret- 

 Concentration * na ' ^ e mm( ^ possesses the power of singling any one of them 

 of attention on out and fastening attention upon it, just as among a number 

 of musical instruments simultaneously played, one, and that 

 perhaps the feeblest, may be selected, and its notes exclusively followed. 

 These phenomena, however, are not dependent upon any peculiarity of 

 construction of any of the organs of sense ; and as the mind can perceive 

 the images of external things, so can it give rise to spectral illusions 

 which may simulate perfectly the aspect of external forms. The anec- 

 dotes of such occurrences which are to be found among all people are not 

 the fabrications commonly supposed. The mind can be readily deceived, 

 even in spite of itself, as the phenomena of the stereoscope prove ; and 

 spectres, having their origin in natural or diseased conditions of the brain, 

 may accurately replace images that have been painted in the eye. It is 

 Sub'ective im sa ^' however, that we may readily distinguish, by means of 

 ages, and test a simple optical test, a true external apparition, if any ex- 

 ists, from a phantom of diseased imagination ; for by press- 

 ing duly with the finger on the ball of one of the eyes, external objects 

 are at once doubled, but it is not so with a mental illusion ; and we may 

 therefore suspect that, even in the best authenticated cases of the ap- 

 pearances of these unnatural forms, had this test been applied, their true 

 character would have been ascertained ; and that, since none of them 

 would have undergone duplication, they would at once have been detect- 

 ed as mere hallucinations of the mind. 



The explanation of the function of vision which I have given on the 

 preceding pages might be termed the calorific hypothesis, since it rests 

 essentially on the fact that the temperature of the receiving screen of the 

 eye is raised by the impinging of light upon it. The result thus far is of 

 a purely physical nature, but it becomes physiological when we farther 

 admit that changes of constitution ensue in the vesicular structure of the 

 retina. These changes are rendered more rapid as the temperature is 



