SENSE OF SIGHT. 449 



lighter and more slender in outline than modern buildings of 

 white stone. M. Le Roux's 1 researches show that the phe- 

 nomenon of irradiation is peculiar to the field of indistinct 

 vision ; it increases with the distance from the yellow spot; 

 the only radiation which occurs in this part of the retina is 

 that produced by the limits of the acuteness of vision. The 

 radiation in the field of indistinct vision is explained by the 

 progressive interval found between the sensitive elements 

 (rods and cones), as we get farther from the yellow spot 

 where the highest degree of condensation takes place. These 

 phenomena ^of irradiation may, in pathological conditions 

 of the brain, as in delirium, increase to such a degree as to 

 completely upset the reason. 



Nearly all the numerous phenomena known by the name 

 of optical illusions may be considered either as instances 

 of the persistence or else of the irradiation of images upon 

 the retina. To these must be added the excitations which 

 take place in the retina itself (subjective images, entoptic 

 perceptions). The principal of these are due to modifications 

 in the circulation. We have seen that the retina contains 

 vessels (pp. 443 and 446) ; these vessels sometimes become 

 congested, and produce compression of the elements of the 

 retina, which, when slight, excites the sensitive membrane, 

 and, when strong, paralyzes it. 



Thus, if we lower and raise the head suddenly, we pro- 

 duce subjective visual sensations, consisting of bright and dark 

 spots which seem to be impressed on the eye. Many cases 

 of blindness are owing to vascular derangement of the 

 retina, which may be easily discovered in the living subject 

 by the use of the ophthalmoscope. 



Looking with the eye into the microscope, especially 

 when there is no object in the field, reveals other entoptic 

 images which are extremely curious; these are muscce voli- 

 tantes or specks, which appear under the form of masses of 

 small and very round globules, all of which have nearly* the 

 same size, and are entangled with some sinuous filaments. 

 Ch. Robin has shown that these images are produced by the 

 projection on the retina of the shadow of the globules and 

 the filaments (elements of mucous tissue or embryo connec- 

 tive tissue), which are suspended in the vitreous body. 2 



One circumstance which has greatly perplexed physiologists 



1 Academie des Sciences. Avril, 1873. 



2 Ch. Robin, " Traite du Microscope." 1871, p. 437. 



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