74 PKACTICAL PHOTO-MICBOGBAPHY 



and other apparatus on geometric slides will be described in a 

 later portion of the book when dealing with photographic 

 cameras. This method when used for the support of the 

 collecting-lens system and other apparatus required for illumin- 

 ating purposes will be found equally efficient, and where cost 

 is a consideration, much more economical. 



A diagram is also appended (Fig. 21 d) showing the 

 arrangement which is necessary when incandescent gas is 

 used as the radiant, or any similar source of light of large 

 area and low intensity. The illustration is sufficiently descrip- 

 tive and requires no further explanation. 



In cases where the cost of the Kohler system of lenses is 

 an objection, a fairly efficient substitute may be arranged by 

 fitting up three ordinary non-achromatised lenses. These may 

 be purchased for two or three shillings each from most 

 opticians. Two plano-convex lenses of about 10 to 12 inches 

 focal length, and one bi-convex of about 18 inches focus will 

 be found suitable. They may in use be set up in the manner 

 shown (Fig. 21), and will be found more efficient than an 

 ordinary bull's-eye condenser. 



