The Microscope. 269 



If the opening in my slide carrier were one and one-eiglitli 

 inches wide, as it ought to be, I would probably use all slides 

 one and three-quarter inches by one inch, such as those sold by 

 the Bausch & Lomb Optical Co., but, as it is, I have been 

 obliged, for some time past, to work against another prejudice, 

 and use narrower slides, and I find a slide three-quarters or 

 seven-eighths of an inch wide ample for most uses, and for all 

 covers, three-eighths to three-quarters of an inch square, and 

 is sufficient for the label ; so what more do we want ? If it is 

 more labeling room, the two sides of the free end of the short 

 slide can be used, thus affording plenty of space for the purpose. 



The labels which I prefer for the purpose I cut out of gummed 

 paper, in slips one-quarter to one-half an inch wide, and two to 

 two and one-quarter inches long, as needed. Moisten and apply 

 around the free end of the slide ; after drying, write the name 

 and number on the upper surface, and the mounting medium 

 date and stain on the lower. In this way I find room for every- 

 thing on the short slide. 



I have heard it objected that the label under the slide 

 places the latter out of level on the stage. This, at first sight, 

 appears true, at least theoretically. But let us see how fine a 

 theory it is. The thickness of a sheet of my gummed paper, 

 measured by a micrometer, is about the one-thousandth of an 

 inch, determined by holding the paper edgewise in a stage for- 

 ceps, under the microscope. In case we are using a power of 

 500 diameters, the field will be about one one-hundredth of an 

 inch across. Now, by proportion, the length of the slide is to 

 the diameter of the field as the thickness of the label is to the 

 inclination of the field, or x. Then, 1^ in. : jl-g in. : : xotfo in. : x ; 

 I X = Tooffoo ; X = Trs'oou of an inch, for the variation from a 

 theoretical true level, or in the breadth of vision with a one- 

 fifth or one-sixth objective ; and if we are using a two-inch, one- 

 tenth of that, which is iihi), is out of level, on account of tlie 

 label on the under side of one end. Where is the instrument 

 maker, however skillful, who can construct a stand the tubes of 

 which shall vary so little as that from a true perpendicular to 

 the stage. The best slides may vary more than that in the 

 thickness of ends. Stands that raise one side of the stage, by 

 the fine adjustment, as do some of the Acme stands of J. W. 

 Queen & Co., disregard this principle in a very much larger de- 



