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



transmitted portion cf radiation is increased, while there is a corre- 

 sponding dimiimtion of the intensity of the reflected portion. This 

 I have been a.ble to verify qualitatively from numerous observations. 

 But in making quantitative measurements many serious difficulties 

 are encountered, owing to the difficulty of maintaining the intensity 

 of radiation, as well as the sensitiveness of the receiver, absolutely 

 constant. 



As regards the first, the intensity of the emitted radiation depends 

 on the efficiency of the secondary spark, and the nature of the spark- 

 ing surface. Keeping the primary current that flows through the 

 Buhmkorff coil constant, the efficacy of the secondary spark is very 

 much affected by the manner in which the contact is broken in the 

 primary circuit. If a vibrating interrupter is used, the break is apt 

 to become irregular ; the torrent of the secondary sparks also spoils 

 the sparking surface of the radiator. For merely qualitative experi- 

 ments the use of a vibrating interrupter is not so very prejudicial, as 

 along with the ineffective discharges there are present some which 

 are oscillatory. But where successive discharges are to give rise to 

 radiation of equal intensity, it becomes necessary to avoid all sources 

 of uncertainty. For these reasons I prefer a single break for the 

 production of a flash of radiation. With some practice it is possible 

 to produce a number of breaks, each of which is effective. If the 

 surface at the break is kept clean, and the break is properly effected, 

 successive flashes of radiation up to a certain number are about 

 equally intense. When the sparking has been taking place for too 

 long a time, the surface no doubt undergoes a deterioration. Bat 

 twenty or thirty successive sparks are equally efficacious when spark- 

 ing takes place between platinum surfaces. The use of a single flash 

 of radiation is preferable on another account. The receiver at each 

 adjustment responds to the very first flash, but becomes less sensi- 

 tive to the subsequent flashes. The conditions of the different experi- 

 ments are maintained similar, when the action on the receiver is due 

 to a single flash of radiation, instead of the accumulated effect of an 

 unknown number of flashes. 



I give below the deflections of the galvanometer produced by four- 

 successive flashes of radiation. 



(1) 115 divisions. 



(2) 122 „ 



(3) 113 



(4) 108 „ 



When very careful adjustments are made, the successive deflec- 

 tions are approximately equal. There are. however, occasional 

 failures, owing either to the fault of the break, or loss of sensitive- 

 ness of the receiver, 



