1026 



SUMMARY OF OUHKENT RESEARCHES RELATING TO 



made of tbis physical law in order to totally reflect all light-rays and 

 cause thcra to pour out on any particular spot. The glass bodies used 

 for tbis purpose are bent into the form of a parabola, and may consist of 

 solid glass or of a glass bell, in which the source of light is at the vertex 

 of the descending branch of the parabola. 



" In the drawings different forms of the instrument are represented : 

 thus fig. 169 shows in elevation and plan a glass boll, which can 

 be used as a lamp-glass. The outer as well as the inner surfaces of this 

 bell are bent on both sides into a parabolic form. The rays from the 

 source of light Z, situated in an opening o in the middle of the bell, are 

 on both sides thrown from one parabolic surface to the other until they 



Fig. 169 



Fig. 172. 



emerge and are dispersed from the lower end. All rays which enter the 

 glass body are totally reflected with the greatest intensity, since each 

 angle of incidence at least amounts to 40^ In the example chosen for 

 the drawing, the side surfaces are not parallel but converge towards a 

 point, in consequence of which the rays are rendered convergent before 

 they emerge from the lower end, and thus the intensity of the emergent 

 beam is heightened. 



" This is also the case in the apparatus represented in fig. 170, which 

 may replace the laryngeal, ophthalmoscopic, &c., mirrors hitherto used. 

 By the use of this apparatus the light-rays are directed upon any desired 

 spot and there uniformly distributed, so that no shadows can occur. 

 The glass body in this case consists of a piece of solid glass bent into 

 a parabolic form, to which a small prism can be attached in order 

 better to see through the beam of light. 



" By fitting into each other several of such parabolic glasses with 

 sides running both parallel and also towards each other, it is possible to 

 direct the light upon any particular spot which cannot be directly 

 illuminated. Various examples of this are shown in figs. 171 and 172." 



