Appendix II 169 



permit the substitution of microscope, polariscope, spectroscopic 

 prism, etc. 



5. Erecting Prism. This is a right-angled prism, mounted 

 immediately in front of the objective in such a way that the 

 hypotenuse face is horizontal, and that the axis of the objective, 

 produced, cuts the vertical line bisecting the right angle of the 

 prism at a height of one-third from the hypotenuse face. It may 

 be shown geometrically that only the lower two-thirds of an erecting 

 prism is used, consequently any mounting other than that suggested 

 will necessitate the lowering of the radiant during an erection. 

 The hypotenuse face may with advantage be silvered. Tabulating 

 the essential characteristics of a practical lantern, then, we have 



1. Interchangeable electric arc and limelight radiants. 



2. Lantern body lined with fireproof material, and sufficiently 



large to permit all the various adjustments being made 

 with ease. A cramped lantern body is a constant source 

 of worry and annoyance to a demonstrator. 



3. A condenser mounted so as to be accessible up to its front 



face. 



4. A free space immediately in front of the condenser for tall 



apparatus. 



5. Some reasonably solid supports for apparatus in front of 



condenser. 



6. Compound achromatic objective properly shielded from 



stray light, and easily dismountable. 



7. Mechanical fine adjustments for objective, and for horizontal 



and vertical motions of radiant. 1 



8. Free space between condenser and objective to admit 



apparatus. 



9. Double stays for objective, in order to allow small platforms 



for apparatus such as a microscope or spectroscopic 

 prism to be placed across the stays when the objective 

 is removed. 



For many years it has been almost impossible to purchase a 

 lantern exhibiting all these essential points, and at a reasonable 

 price a price which would enable it to be purchased for general 

 class-room use. The nearest approach was one placed upon the 

 market by Messrs. Newton, of Fleet Street. It was fitted with 

 arc lamp only, which was not interchangeable with limelight, 

 and it had a single stay carrying the optical front. This, while 



1 A perfectly serviceable method of horizontal adjustment is that of a 

 screwed sector, mounted on a false base. 



