lOG 



SUMMARY OF CUHEEMT EESEARCHES RELATING TO 



Fig. 24 shows in plan the arrangements for megascopic projection 

 The lamp is set horizontally as for microscopic projection, and the mirror 

 Spi is rotated OO'^ about a horizontal axis. The object j» is placed oppo- 

 site the objective Obj and mirror Sp, which are inserted, as previously 



Fig. 24. 



described, into an aperture of the right-hand side-wall. The screen 

 pictures are inverted, but are corrrect laterally. 



Critical Illumination.*— A. W. Blacklock says : " I find that I get 

 the best definition with high powers when the object is illuminated in 

 the way I will now describe. In the diagram (fig. 25) A is the objective, 

 B is the slide, C is the substage condenser, D is the iris stop, E is a 



plano-convex lens about an inch in diameter (the field-lens of the Huy- 

 ghenian part of the eyepiece of an old telescope), F is a metal plate with 

 a small liole in its centre, and G is the flame of a paraffin lamp. The 

 plate F is mounted on a slider, so that its distance from E can be adjusted 

 to the solar focal length of E. The central hole is J-in. in diameter, 

 and there is a metal plate which slides over it and which has three smaller 

 holes of different sizes, either of which can be used as desired. The 

 lamp G was either an ordinary flat-wick paraffin lamp or a two-wick 

 lamp with flattened chimney, placed with the flat surface of the flame 

 towards the hole in F, or an Argand gas-burner, or a Welsbach incan- 



♦ English Mechanic, xciv. (1911) p. 370 (1 fig.). 



