SIGHT AND THE VISUAL ORGAN. 469 







-perceptible, either in certain effects of light, or on a peculiar exertion in 

 the straining of the attention. By a simple experiment it is possible to 

 make every person acquainted with these guests of his field of view, 

 the so-called muscce volitantes ; only, one must be prepared for those 

 formerly overlooked, but once honored with attention, never again 

 stirring from their post. 



In order, likewise, to preserve space for the play of these variations 

 in the form of the lens, in the act of adjustment, it was necessary to 

 surround the lens with a liquid or elastic medium. That the aqueous 

 humor bears a part in moistening the cornea, has already been stated, 

 but in the voluminous vitreous humor we behold the regulator of the 

 shape and tension of the eye. Such an auxiliary medium is of urgent 

 necessity to keep up the regularity of refraction, the quality of tension 

 in the retina as a sensitive surface, and the action of the optic nerve. 

 Some years ago, I was happy in being able to demonstrate that a com- 

 prehensive range of diseased conditions and also blindness, the causes 

 of which had been successively sought for in different parts of the eye, 

 simply arose from too great a tension being exercised by the vitreous 

 humor a discovery which imparted so much the more pleasure, as 

 a suitable remedy was likewise at hand. 



Let us imprint on our memories, from the model of a magnified eye, 

 the positions and dimensions of the different parts. For the present, I 

 begin with the cord of the optic nerve, which, as you perceive, does not 

 enter the sclerotica exactly opposite to the cornea. This part, as well 

 as the larger posterior chamber of the eye, is embedded in the orbit, 

 and is therefore not visible externally ; while, on the other hand, be- 

 tween the lids, you remark the white of the eye, which is the anterior 

 chamber of the choroid ; next the transparent cornea ; and away 

 through it the colored iris, with the pupil in the centre. 



The black appearance of the latter used to be ascribed solely to the 

 dark layer formed by the choroid for the interior of the eye, absorbing 

 all the light. More patient and minute investigations have, however, 

 proved that the pupil derives its blackness only partially from the 

 above circumstance, and mostly from the refraction of the light. 

 Helmholtz has succeeded in banishing that darkness from the pupil of 

 the human eye. By a simple arrangement, called the speculum oculi, 

 he uses the light which is reflected from the deeper parts of the eye to 

 illuminate the whole of the interior, as also the image itself projected 

 on the retina. This invention exercises an influence not only on the 

 peculiar branch of the oculist, but likewise on the broad field of medi- 

 cal investigation, seeing it affords an insight into the optic nerve, a 

 direct process of the brain, and other structures, which, along with 

 their analogies, were hidden from observation in the body. 



The dimensions of the eyeball among strong-sighted individuals is 

 more equal than you perhaps imagine. The apparent differences of 

 size lie almost exclusively in the shape of the slit of the lid. If it has 



