CH. IX] 



TROUBLES WITH MICRO-PROJECTION 



301 



removed. For projection the mirror must be in place to reflect the light along 

 the axis as for lantern slides. The change to the projection of opaque objects 

 is almost instantaneous, but for lantern-slide projection the projection micro- 

 scope must be removed and the lantern-slide objective put in place, but^the 

 apparatus is so constructed that this is easily accomplished. 



FIG. 166. IMPROVED, COLLEGE, BENCH LANTERN ARRANGED FOR 



M ICRO-PROJECTION. 

 (Cut loaned by the Mclntosh Stereopticon Co.). 



The optical bench consists of a long baseboard with the two guide rods 

 supported by three brackets. 



As each part is independent it can be changed in position or entirely removed 

 and other apparatus put in its place, thus giving great flexibility. 



TROUBLES WITH THE PROJECTION MICROSCOPE 



435. The source of troubles with the projection microscope 

 are mainly the same as with the magic lantern. These have been 

 fully discussed at the end of Chapter I ( 62-98). See i28a for 

 the blowing of fuses with the arc lamp on the house system. 



The special troubles with the projection microscope are almost 

 wholly due to the smallness of the lenses necessary for micro-pro- 

 jection; and as the foci of these lenses are relatively short, slight 

 changes in the position of one of the elements of the apparatus, and 

 slight deviations from the true axis produce correspondingly great 

 effects. It is necessary to be more exact in micro-projection, but 

 the great fundamental principles are exactly as for the magic 

 lantern. 



436. Insufficient illumination on the screen. Besides those 

 given in Chapter I the following may be causes : 

 i. Too large a screen image may be attempted. 



