46 THE MICROSCOPE. [March, 



PRACTICAL SUGGESTIONS. 



To Prevent the Reddening of Canada Balsam. — The ten- 

 dency of Canada balsam to become red may be checked, and the 

 balsam bleached, by the addition to the solution of a few crystals 

 of pure metallic copper, precipitated from a copper sulphate solution 

 by any of the ordinary methods. This process originated with 

 the late Allen Y. Moore, of Cleveland, Ohio, and was the result 

 of accident. What the philosophy of it is we do not know, as 

 the copper crystals do not seem to be changed in any manner, 

 even after long immersion in the solution of balsam. — National 

 Druggist. Dec. I. '<?2. 



Bleaching. — Many drug sections require to be bleached be- 

 fore staining, but all traces of the bleaching- material must be 

 removed also. This, however, destroys and removes all the cell- 

 contents. 



Silk, wool, and similar fibres may be bleached by sodium 

 superoxide(Na 2 0_,) . which is soluble in water. The liquid is 

 neutral. The bleaching requires from 2 to 3 hours. 



SCIENCE-GOSSIP. 



To Measure the Magnifying power of a Microscope. — A 



camera lucida of some sort should be placed over the eyepiece to 

 obtain the best result. A very simple one. indeed, may be made 

 of a square of thin covering-glass tilted at an angle of 45 to the 

 optic axis of the instrument. If with this arrangement the mi- 

 croscope tube be placed in a horizontal position, and we look 

 down onto our piece of thin glass, the field will apparently be 

 projected onto the table. Under these circumstances, the eye-end 

 should be exactly 10 inches above its surface, and the whole in- 

 strument must be propped up until it is at this distance. We 

 now require a stage-micrometer and a rule accui'ately divided to 

 tenths of an inch. This latter we lay upon the table, 10 inches 

 from the eyepiece, so that the magnified image of the divisions of 

 the stage-micrometer shall be superposed on those of the rule. 

 We then note how many divisions of the stage-micrometer cor- 

 respond with any given number on our rule, and, dividing the 

 latter by the former, obtain the magnifying power. For exam- 

 ple, suppose that our stage-micrometer is divided to .001 inch, 

 and suppose that 3 of its divisions = .003 inch exactly corre- 

 spond to 0.6 inch on the rule. Then, dividing 0.6 by 0.003. 

 we get 200 as the magnifying power with that object-glass, eye- 

 piece, and length of tube. Had four divisions coincided with \ 

 inch on our rule, the microscope would have magnified 125 



