CH. IX] MICRO-PROJECTION WITH LIME LIGHT 287 



distance from one another are as for the electric light described 

 above. 



As the spot of light must remain in exactly the same place to be 

 received by the small lenses of the projection objective, it is neces- 

 sary to regulate the hand heliostats oftener than for the magic 

 lantern. 



It may also be necessary to make slight corrections in the mirror 

 of the clock-driven heliostat from time to time. The law is : The 

 axial ray must correspond with the optic axis of the apparatus. 



421. Use of a water-cell. The radiant energy of the sun is 

 so great that a water-cell to remove as much of it as possible except 

 the luminous part ( 844) is as desirable as with the electric light. 

 It is also desirable to have a specimen cooler (fig. 121). 



PROJECTION MICROSCOPE WITH THE LIME LIGHT 



422. The management of the lime light for the projection 

 microscope is exactly as for the magic lantern (see 1 63 , 1 64) , only 

 more attention will be necessary to keep the best possible light all 

 the time. The image of the luminous spot should be focused on 

 the hood of the objective as for the electric arc. While there is 

 not so much danger from overheating as with the electric light or 

 sunlight, it is desirable to use a large water-cell. The stage cooler is 

 also an advantage. For the correct form of a condenser see 363. 



As the intrinsic brilliancy of the lime light is less than that of 

 sunlight or the electric light one must not expect so much of it as 

 of them. 



423. Other sources of light are insufficient to give good 

 micro-projection except in a very limited degree, and for some 

 special purposes. See under drawing, Ch. X, 463. 



HOME-MADE PROJECTION APPARATUS 



424. Projection table. For all kinds of projection the table 

 should be of convenient height, so that the operator can stand dur- 

 ing the exhibition. A height of 100 centimeters (40 inches) is 

 suitable for most persons. The size of the top varies greatly with 



