PRACTICAL PHOTO-MICROGRAPHY. 



71 



more than 125 deg. is required, it answers fairly well in this 

 capacity. 



Another method of compromising has been much used by 

 the writer for very low power work, and even under certain 

 circumstances, for high power work ; it answers as a makeshift 

 for the low power work, but is not recommended for high 

 power where, in fact, it is not necessary. The arrangement is 

 shown in figure 26. 



G B 



FIG. 26. 



Here R is the radiant, B a bull's eye parallelizing ff& rays 

 upon a disc of very finely ground glass G, the bull's eye and 

 disc being so fitted that they can be fixed as a whole piece of 

 apparatus, in front of the radiant. Reference to figure 1 6 will 

 explain the fitting of this to the front of the lantern ; S, in 

 figure 26, being a pinch screw by which the apparatus is fast- 

 ened to the front. Here G becomes practically the radiant, 

 and the writer hoped great things for this arrangement, until 

 he found that he could not accurately focus the ground glass, 

 even when oiled, on his object without getting an image of the 

 grain of the ground glass, which was fatal, of course, to accurate 

 focussing of this radiant. But for the lowest power work, where 

 the angular aperture of the objective (as 3-inch, 4-inch, etc.) is 

 very low, and where the use of a condenser is forbidden, this 

 apparatus is strongly recommended and often used by the 

 writer. It might appear that by using, in place of the piano 

 convex B, a double convex, a small disc of very brilliant light 

 would be obtained on G, the size regulated by sliding G to 

 and from J3 5 and that this small disc would be very valuable 



