ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 455 



of being permanent, not aifected by beat or cold, and being cheap and 

 easily worked. Tinfoil '03 in. thick is the more generally useful. 

 The rings can be punched out by two pimches of different size. It is 

 almost impossible to make the two circles concentric and smooth, so 

 after a number are punched out they should be placed on a mandrel 

 fitting the inner circle exactly, and putting it in a lathe, tnrn down the 

 outside perfectly true. If deeper cells are wanted it is easy to cement 

 any number of the rings together. 



Professor A. McCalla finds it easier, after punching out the inner 

 hole, to cut the rings apart with scissors, without attempting to make 

 them round on the outside as lathe cutting does that perfectly. It will 

 probably be a good plan to fit rotary cutters on the lathe to cut out 

 several consecutive circles at once somewhat as leather washers are cut. 



Ivory Drop-Black.* — Mr, E. Graham uses this material f as a 

 background for all opaque mounts. It makes when properly applied 

 a beautiful smooth surface. 



Press a small quantity of the colour into a one-ounce wide-mouth 

 bottle, and thin it sufficiently with fresh turpentine. The sKde being 

 on the turntable, apply the colour with a brush. If the colour is too 

 thick, it will be found that it cannot be smoothly spread, and that it 

 will dry in ridges. If too .thin it will be found necessary to make 

 several applications. If it is necessary, a second application can be 

 made within fifteen or twenty minutes. 



Selection of Cover-glass.t — Dr. J. E. Smith tries to confine 

 himself to three thicknesses of cover-glass, namely l-70th in., 

 l-120th in., and l-200th in. These may respectively be denominated 

 as thick, medium, and thin. It is a matter of the fii-st importance 

 that those working first-class objectives should be well posted as to 

 the thickness of cover employed, and yet this telling point has been 

 utterly lost sight of in the books. For example : by knowing the 

 thickness of the cover, one is enabled to approximately adjust the ob- 

 jective at sight, and thus save time. He has thousands of mnimted 

 objects in his cabinets, and every cover has been measured with all 

 the accuracy obtainable. Those who have long had their attention 

 called to this matter can, by dint of practice thus obtained, tell 

 closely the thickness of the cover by simply /eeZ/H^ it; and this, he 

 assures the novice, is an accomplishment worth having. 



" To take a case from practice : Suppose I desired to examine a 

 brand-new mount. Let it be a difficult diatom this time. First, I 

 run my finger over the cover, and instantly discover that it is a thin 

 one, say about like those used on the Moller plates. Now, if I elect 

 to use the l-6th objective, I know that this cover is too thin for water 

 immersion ; hence glycerine is chosen. I know, too, that over such a 

 cover, and with the glycerine intermedium, the objective will correct 

 some three or four divisions from closed, therefore the collar is at 



* Amer. Mon. Mier. Joum., ii. (1881) p. 113. 



t The " XXX ivory drop-black " of Sherwin Williams and Co., of Cleveland, 

 Ohio, put lip in collapsible tubes and ground in japan, and not in oil, which will 

 not do, as it always dries with more or less gloss. 



X ' How to see with the Microscope,' 1880, pp. 213-15. 



