42 



FOCUSING ON THE GROUND GLASS. 



Much has been written about the proper focusing of the object. Our 

 experience leads me to conclude that the real difficulty has always been 

 that the machinery of the microscope was not sufficiently accurate, its 

 parts were not sufficiently firm relatively, the microscope itself was not 

 sufficiently supported against damaging strains and jars, and its fine 

 adjustment screw was not sufficiently fine; we need nothing but a fine 

 ground glass and the unaided eye for correct focusing; a, plate glass and a 

 focusing lens are generally recommended: they are scarcely a help; the 

 difficulty vanishes with such stable and delicate machinery as puts control 

 entirely in the hands of the one focusing. 



POSITION OF THE SENSITIVE PLATE. 



A pure scarecrow of the books is the oft repeated necessity of having 

 the sensitive plate take the exact place of the ground glass; some one 

 must have concluded that a want of coincidence in this respect spoiled 

 his plates, and other essay mongers must have copied the conclusions. 

 Doubtless he and they had spoiled plates, but the cause was not here; 

 a variation of a quarter of an inch makes a perceptible difference in mag- 

 nification, but not in sharpness, and no instrument probably ever varied 

 so much as this. 



EXPOSURE. 



The time of exposure depends on so many things it is not possible to 

 give any rules: The source of the light, its intensity, the number and 

 character of the condensers, the number and character of the color 

 screens, the width of the diaphragms, the character of the object, the 

 objective and eyepiece used, the sensitiveness of the plate, and the fresh- 

 ness and strength of the developer, all materially affect the time. Any 

 one can find out the time necessary by a few trials provided he under- 

 stands development and is a good judge of a negative. If he has not these 

 accomplishments he never can tell. Some kind of shutter with which 

 to accurately measure fractions of a second is so useful as almost to be 

 necessary in getting the right exposure; placing a ground glass in the 

 path of the light near its source will multiply the time of exposure some 

 twenty-five times and would be necessary in the absence of a shutter. 



