296 OUTPOSTS OF THE INTELLIGENCE SERVICE 



candle, held near the temple, allowed to fall obliquely on the 

 sclera, the shadows of the vessels are projected on to the retina. 

 The eyes should have their accommodation fully relaxed and a 

 suitable dark screen be placed in a plane parallel to the antero- 

 posterior plane of the head. Under these conditions the vascular 

 network of the retina appears, in a highly magnified form, as if 

 projected on the screen. If the light is focused on the sclera by 

 means of a biconvex lens, or if a brighter source of light is used, 

 flashes of light following definite paths are seen. These flashes are 

 produced every time a transparent leucocyte passes across the 

 path of light. The interposition of a dark blue-violet light filter 



will make this phenomenon clearer. 

 The red cells of the blood absorb the 

 blue rays and appear as an almost 

 continuous shadowy stream broken 

 here and there by a leucocytic flash. 

 By holding the breath one alters the 

 rate of blood-flow, and so produces a 

 definite alteration in the rate at which 

 these flashes appear (Fig, 80). 



Arc Phenomenon. Under suitable 

 conditions nerve bundles lying on the 

 upper and lower borders of the tem- 

 poral half of the macula and extending 

 to the optic disc may be seen (Ellis). 

 If one is looking at a distant street 

 lamp (a doctor's red lamp serves well) 

 in the evening, when it is possible to 

 use a wall in shadow as a dark screen 

 for projection purposes, two arcs of a 

 bluish colour are seen, set with their concavities facing. The oval 

 space between the arcs is filled with a bluish haze. If the distant 

 light is feeble or poor in the more refrangible red rays, the arcs 

 may be difficult to see and only the haze be perceptible. 



Muscae Volitantes. These motes, which move steadily down- 

 wards as the eye is directed upwards, are parts of a diffraction 

 pattern produced by substances in the vitreous body. 



Ophthalmoscopy. 



The interior of another person's eye may be viewed quite readily 

 by suitable methods. 



When we look at a person's eye the pupil appears perfectly 

 black. Nothing can be seen of the interior because it is feebly 

 illuminated compared with the outside world. If we could light 



ti M 



Fig. 80. — Diagram of the path of the rays 



of hght in the formation of Purkinje's 



figures. 



F represents a retinal vessel. When 

 this is illuminated from A, a shadow is 

 formed on the sensitive layer of the retina 

 at a'. This is jrojected along a line 

 passing through the optic axis and 

 appears to come from a joint (a") on the 

 screen. On moving the light from A to 

 £, the image of the vessel on the screen 

 appears to move from a" to b" , 



