ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



7 01 



ent focus without special adaptation. Hence the degree of magnification 

 can only be varied by altering the distance between the apparatus (which 

 is placed on casters) and the screen. With the small search-light the 

 magnification of a uniformly illuminated area of 9 sq. iu. is magnified 

 9 diameters ; but smaller objects may be maguitied up to 25 diameters. 

 With the larger lamp the magnification varies from 14 to 37 diameters. 

 An opaque screen is recommended for the projection and may be pre- 

 pared as follows : — A wall space, or a paper or linen screen, is painted 



Fig. 132. 



with white zinc, prepared with water and size, and dusted over with 

 powdered chalk just before the coat of paint becomes dry. Such a 

 colouring can be easily renewed when necessary. 



When reflected light is used the light emitted by the crater of the 

 positive carbon falls upon the parabolic mirror of the lamp and is thence 

 reflected in the shape of a nearly cylindrical pencil. It next passes 

 through the cooling chamber (which is filled with water and performs 

 the function of absorbing heat rays), then strikes a mirror and is by it 

 reflected obliquely through the diaphragm and upon the object inline- 



