70 THE USE OF THE MICROSCOPE 



what the clouds do to the sun's hght. There is apparently 

 no proved foundation for the old assumption that the 

 differences of phase in adjacent points of the direct flame 

 are of importance for microscopical vision. A disc of 

 finely double-ground glass is put in front of the source 

 of light, and is diaphragmed to fit the field of view. 

 Flashed opal glass is said to be good (Hartridge) ; but, in 

 the writer's experience, it cuts off too much light, and is 

 brownish yellow if ground thin. The glass should be 

 finely ground. The doubly ground glass disc may be 

 rubbed smoothly on both sides, when warm, with a nearly 

 saturated solution of magnesium sulphate, which is then 

 polished ofT, so as only to fill the scratches. When hot, 

 this forms an almost imperceptible powdery deposit, shut- 

 ting off any direct light, and leaving only diffracted light. 

 The fine glass powder formed by grinding two glass discs 

 together with water and a minimum of fine carborundum 

 flour does as well as the magnesium sulphate, if the discs 

 are merely wiped, not washed. Two such freshly ground 

 faces may be put together and fastened (Zeiss). (Perhaps 

 a good matt surface could be had by using hydrofluoric 

 acid on clean plate glass.) A C-Mazda bulb, matt inside, 

 can be used, with diaphragms in front, even for high 

 powers, if of about 100 watts, at 110 to 120 volts. A test 

 for the absence of much direct light through ground glass 

 is the putting of a condensing lens at such a distance from 

 the source (C-Mazda lamp) that it forms an image of the 

 source on a screen. On interposing the ground glass, 

 every trace of an image should disappear in a uniformly 

 diffused light. In this way, it may be roughly seen whether 

 the ground glass needs grinding on both sides, or whether 

 one ground surface will suffice. A single condensing lens 

 may be placed between the lamp and the ground glass, 

 to increase the illumination on a circle of the latter. For 

 the high powers, a small strong condensing lens may be 

 turned in, producing an intensely illuminated small circle at 

 the center of the ground-glass disc. But the thin metal 

 diaphragm before the ground glass, of such a size as to make 



