212 OPTICAL PROJECTION 



moved, a central parallel beam proceeds from the parallelising 

 lens on the alum-cell to the screen. Thenceforth the proce- 

 dure is precisely that already described for the oxy-hydrogen 

 apparatus, only racking the front lantern condenser, or 

 screwing the lamp -stand to and fro, instead of moving the 

 lime-tray. Good centering is all-important. The substage 

 condenser, in critical cases, must then be carefully fociissed on 

 the slide. And finally (the demonstrator here using a bin- 

 ocular glass unless his sight is very keen) the objective itself 

 being focussed as accurately as possible, and the required 

 point in the slide brought into the centre of the field, the iris 

 diaphragm is to be adjusted till the best effect is found. 



In using the projection microscope with the electric light, 

 it is particularly necessary to remove any air-bubbles on the 

 sides of the alum-trough from time to time. The obstruction 

 to the light is not of so much consequence, but they interfere 

 with a truly parallel beam, and usually focus upon the screen 

 when the adjustment of the light is otherwise at its very 

 best. 



CHAPTER XIV 



DEMONSTRATIONS OF APPARATUS, AND IN MECHANICAL 

 AND MOLECULAR PHYSICS. 



107. Projection of Apparatus. Referring back for a 

 moment to fig. 81, page 154, it will be seen that a gold-leaf 

 electroscope, placed in the field of the lantern, upon a small 

 table adjustable both for height and distance from the con- 

 densers, takes the place of an ordinary diagram or slide, and 

 is then projected upon the screen in precisely the same way. 

 There are some limitations and also some difficulties about 

 such projections, which deserve a word or two. 



