308 The Microscope. 



it slowly over the object after having placed one edge against 

 the slide, supporting the opposite margin by the needle. In this 

 way the cover may be very gradually depressed or as slowly 

 raised. 



GLASS SLIPS. 



Slips should be of the best and whitest glass, without bubbles, 

 striae, scratches or flaws of any kind. They should also be so 

 smooth and so perfectly flat that when they are pressed together 

 the}^ will adhere to each other. The best size is that commonly 

 used in this country and in England, three inches long by one 

 inch wide. .Many cheap slides of French or German origin meas- 

 ure only two and three-eighths by five-eighths. These are entirely 

 too small, and should never be used. In Germany, again, slips 

 are sometimes made measuring ©ne and seven-eighths by one 

 and one-eighth ; occasional!}' three and one-half by one and one- 

 half inches. There is little advantage in using these uncommon 

 dimensions. They cannot be conveniently stored away in any 

 ordinary cabinet, and they are not so easily handled as are those 

 of the standard size. 



The edges should be smoothly ground. Many of the best 

 have the edges also polished, and this is a pleasant thing to look 

 at, but not a necessit3^ Roughl}' cut margins are to be avoided 

 if the microscopist cares anj'^thing for a whole skin on his finger 

 ends, and an unmarred stage to his microscope. 



Slips immediately from the dealers are usually as greatly in 

 need of cleaning as are thin covers. Man}- microscopists im- 

 merse them for several days in a solution of bichromate of 

 potassium, two ounces ; sulphuric acid, three fluid ounces, and 

 water, twenty-five ounces. I have never found any of these 

 cleansing fluids necessary. A good rubbing with a soft cloth, 

 a thorough rinsing in warm water, and the application of a little 

 human saliva, are usually all that is needed. The fastidious 

 microscopist is not supposed to know the worth of his saliva 

 for any but digestive purposes, yet he will find it of peculiar 

 value in cleaning microscopical slips and thin glass. It will 

 sometimes accomplish what acid will fail to do. 



The thickness of the slips should be considered when pur- 

 chasing them. I should advise the beginner to avoid the thick 

 as unnecessarily weighty and cumbersome, and to select the 

 medium or the thin. If no sub-stage condenser is to be used, 

 then the medium will answer every purpose, and even when 



