CORRESPONDENCE 115 



to the other is referred to as if it were the actual source of light. This, 

 of course, is not the case, the source being in the case of continuous current, 

 the crater on the positive carbon. The length of arc is also arbitrarily laid 

 down as being one-eighth of an inch ; but this again would vary with the 

 amount of current and the size of the carbons." 



With regard to the statements made, I cannot find any reason for the 

 first, and if your reviewer will turn to the bottom of page 45 he will read : 

 " The light from the continuous current Arc Lamps comes chiefly from this 

 upper or positive carbon, which ' craters ' as it is used." 



It is true that I state later on that the current is conducted across the 

 Arc by a bridge of white-hot carbon particles, but there is no statement what- 

 ever that this is the main source of light. 



With regard to the length of the Arc being arbitrarily laid down as one- 

 eighth of an inch, this is distinctly stated as being approximate only, and as 

 a guide for the amateur in lantern work it is sufficiently near. 



Your reviewer also expresses his regret that Optical Bench Lanterns 

 are not referred to, but he seems to have overlooked the fact, although he 

 refers to it, that the book is being published in two parts. The one now 

 under review deals with Optical Lanterns for ordinary slide demonstration 

 only, and the Optical Bench type, belonging to the class known as Science 

 Lanterns, will be dealt with in their right place. 



I am sorry to have to make these comments, but I think it is a great 

 pity that a reviewer should contribute criticisms which a little more careful 

 reading of the book would have avoided. 



Yours faithfully, 



R. S. Wright. 



May 17, 1921. 



