68 



THE AMEKICAN MONTHLY 



[April, 



lenses than with others, the differ- 

 ence increasing with the increase of 

 power, but with, all it is so great, 

 that such scales as we have referred 

 to, would be worse than useless. 

 Accuracy can only be obtained by 

 using the micrometer with the 

 collar-adjustment at the same point 

 at which the object sketched has 

 been examined, unless, indeed, one 

 were willing to take the trouble of 

 compiling a table for each objective, 

 with the magnification noted for 

 each division of the collar through 

 the whole range from " uncovered " 

 to " covered." J. D. C. 



Cover-glasses. 



There seems to be no good reason 

 why the dealers should not furnish 

 cover-glasses accurately assorted as 

 to thickness. This does not appear 

 to be done. Our most reliable 

 opticians have two or three grades, 

 but these are by no means carefully 

 assorted ; and one who buys in the 

 expectation of getting the thinnest 

 covers at even a relatively high price, 

 is apt to find himself disappointed. 



I have recently measured three 

 lots of covers by means of a good 

 "lever-of -contact", measuring to 

 thousandths of an inch, and give the 

 results below. The covers were 

 obtained from a first-class house, 

 and my experience corresponds 

 with that of friends. The method 

 used was to put the covers in 

 separate piles and measure the height 

 of the several piles or columns. 



First lot, 2 oz. marked y^^ to ^^-inch, 

 making a column 68 """■ high, price $1.75 

 per once. 

 Those .005 in. thick, measured !'»"'• in height. 



.006 " " 13 



.007 " " 18 



.008 " " 12 



.009 " " 6 



.010 " " 6 



.011-.013 " " 8 



.014-.016 " " 4 



ggmm 



From the foregoing it will be 

 seen that less than one-third of the 

 covers came within the limits 

 marked by the label, and two-thirds 

 belonged to the cheaper grades in 

 the catalogues. Only one sixty- 

 eighth were of ^-g- or .005 of an 

 inch in thickness, and yet that 

 thickness of cover is a very ordinary 

 one for use with moderately high 

 powers. By far the greater part of 

 the whole are only fit for covering 

 opaque objects, or objects for low 

 powers. 



To say nothing of the overcharge 

 for an article which is not what it 

 purports to be, there is a more 

 general consideration of interest to 

 all microscopists. We ought to be 

 able to purchase readily, any thick- 

 ness of cover desired, and to be able 

 to rely on its measurement. The 

 lever-of-contact is a somewhat ex- 

 pensive instrument, and in an hour 

 or two one could measure all the 

 covers he would use in years. The 

 dealer should do this work and give 

 us the assorted covers in thoroughly 

 accurate shape, so that one could 

 order any thickness from .002 " 

 upward. 



The second and third lots, meas- 

 ured with the one analyzed above, 

 were labelled respectively T-o"o"'to 

 ^\^ " and -^ " to T^'/. ' Number 

 two contained none so thick as some 

 that were in number one, and both 

 the second and third lots answered 

 very well to their labels. 



K. B., Jr. 



The Simplest Forms of Lite.* 



BY B. EYFERTH. 



Infusoria. — {Continued^ 



The bodies of Lif usoria consist of 

 a soft, colorless, granular paren- 

 chyma, which passes into a some 



* Translated from the German, by the Edi- 



