304 Dr. J. C. Bose. On the Influence of the Thickness of 



Fig. 2.— L is the lens to render the incident beam parallel ; P, P', are the right- 

 angled isosceles prisms ; A and B are the two positions of the receiver. The 

 receiver-tube is not shown in the diagram. 



tinuous. This is the case only when the two prisms are made of the 

 same substance. If the second prism be made of sulphur, or of any 

 other substance which has either a lower or a higher refractive index, 

 there is always found a reflected portion even when the two prisms 

 are in contact. 



Another interesting observation can be made by separating the 

 prisms for total reflection. There would now be no transmitted por- 

 tion. But if a thin piece of cardboard or any other refracting sub- 

 stance be now interposed in the air-space, a portion of the radiation 

 will be found to be transmitted, and it will be found necessary to 

 separate the prisms farther to reduce the transmitted portion to 

 zero. 



Having given a general account of the experiments, I shall now 

 describe the method of procedure. The radiator tube was provided 

 with an ordinary lens whose focal distance for electric radiation is 

 about 4 cm. The beam thus rendered approximately parallel fell 

 perpendicularly on tlie face of the glass prism. The two prisms 

 were made by cutting a cube of glass — an ordinary paper weight — 

 across a diagonal. The size of the cube was 4'5 cm. on each side * 

 One prism was fixed on the spectrometer circle ; the other could be 

 moved so as to vary the thickness of the interposed air-space 

 between the two sections very gradually. The separation was simply 

 effected by means of ordinary cards. The cards used were of uniform 

 thickness, each card being 0'45 mm. in thickness. A certain 

 number of cards were taken and placed between the prisms with, 

 their surfaces in contact with the hypotenuses. The cards were 

 then carefully withdrawn, leaving the prisms separated by a thick- 

 ness of air equal to the thickness of the given number of cards. It 

 would, of course, be an improvement to have a micrometer screw by 

 which the thickness may be gradually increased. 



* Larger prisms would have been preferred, had they been available. The- 

 prisms after cutting were found to be approximately isosceles, the angles being 

 90°, 46°, and 44°. 



