ON PROJECTION 13 



illumination of the ' disc ' (or lighted area) on the screen. 

 And in microscopic projection, where definition in the image 

 is so much more severely tested, it often becomes necessary 

 practically to ' focus ' the light itself upon the object, in order 

 that the rays diverging again from that focus as from a new 

 point, and the image-forming rays from the object itself, may 

 practically coincide. 



Such are the elementary principles and conditions of 

 Optical Projection. Simple as they may seem, for want solely 

 of a due understanding of them, many demonstrators never 

 half develop the powers of their apparatus. This does not 

 apply only to so-called itinerant lecturers ; for I have repeatedly 

 seen polarisation and other physical phenomena projected, 

 with the most elaborate electric -light apparatus, at what are 

 considered the very head-quarters and principal arenas of 

 scientific exposition, in a manner inferior to what I had been 

 accustomed to obtain with only oxy-hydrogen illumination. 

 These principles are the essential key to the whole of what 

 follows; and both excellence of apparatus, and success in 

 using it, depend upon their being thoroughly grasped, in the 

 first place by the optician, and in the second place by the 

 operator who uses the apparatus which the other has con- 

 structed. 



CHAPTER II 



THE PABTS OF A LANTEEN 



A LANTEEN is an optical apparatus so arranged, with all its 

 parts approximately fixed in their places, that pictures or 

 apparatus can be exhibited on a screen with the least and most 

 convenient manipulation. In this place we will consider only 

 the exhibition lantern, for the projection of slides or diagrams, 

 leaving other apparatus for separate consideration in a 

 chapter devoted to experimental lanterns. 



