766 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ble, because the retina is illuminated by light coming from all parts of the 

 pupil, and with such a broad source of light no object, unless it is a very large 

 one or one lying very near the back of the eye, can cast a shadow on the retina. 

 Such shadows can, however, be made apparent by allowing the media of the 

 eye to be traversed by parallel rays of light. This can be accomplished by 

 holding a small polished sphere e.g. the steel head of a shawl-pin illuminated 

 by sunlight or strong artificial light in the anterior focus of the eye i. e. 

 about 22 millimeters in front of the cornea, or by placing a dark screen with a 

 pin-hole in it in the same position between the eye and a source of uniform 

 diffused light, such as the sky or the porcelain shade of a student lamp. In 

 either case the rays of light diverging from the minute source will be refracted 

 into parallelism by the media of the eye, and will produce the sensation of a 

 circle of diffused light, the size of which will depend upon the amount of dila- 

 tation of the pupil. "Within this circle of light will be seen the shadows of any 

 opaque substances that may be present in the media of the eye. These shadows, 

 being cast by parallel rays, will be of the same size as the objects themselves, 

 as is shown diagrammatically in Figure 229, in which A represents a source 



FIG. 229. Showing the method of studying intraocular images (Helmholtz). 



of light at the anterior focus of the eye, and b an opacity in the vitreous humor 

 casting a shadow B of the same size as itself upon the retina. It is evident that 

 if the source of light A is moved from side to side the various opacities will be 

 displaced relatively to the circle of light surrounding them by an amount de- 

 pending upon the distance of the opacities from the retina. A study of these 

 displacements will therefore afford a means of determining the position of the 

 opacities within the media of the eye. 



Muscse Volitantes. Among the objects to be seen in thus examining the 

 eye the most conspicuous are those known as the muscce volitantes. These pre- 

 sent themselves in the form of beads, either singly or in groups, or of streaks, 

 patches, and granules. They have an almost constant floating motion, which 

 is increased by the movements of the eye and head. They usually avoid the 

 line of vision, floating away when an attempt is made to fix the sight upon 

 them. When the eye is directed vertically, however, they sometimes place 

 themselves directly in line with the object looked at. If the intraocular object 

 is at the same time sufficiently near the back of the eye to cast a shadow which 

 is visible without the use of the focal illumination, some inconvenience may 

 thus be caused in using a vertical microscope. 



A study of the motions of the muscce volitantes makes it evident that the 



