1880.] 



MICKOSCOPICAL JOUENAL. 



67 



and burning up. This ash, when 

 examined under a high-power, 

 revealed the valves of the contained 

 diatoms in an almost perfect state 

 of transparency. The water remain- 

 ing over from the first operation 

 was allowed to stand twenty-four 

 hours, when all of the fine particles 

 in suspension settled do-vtn to the 

 bottom, and were easily collected, 

 dried, and burned for the diatoms. 

 Samples of the material secured by 

 the above process will be sent to any 

 one, gratis ; my address is in the 

 " Exchange '* column. 



A quick way of getting marine 

 forms of diatomacese is to take a 

 peck of fresh oyster-shells, and 

 brush the back of each one into a 

 basin of water ; as this fluid is with- 

 out body, it would be well to cut 

 up some cotton into tufts and 

 immerse it in the fluid, which will 

 make the product suitable to be 

 dried and ignited, as it will take 

 fire at a red-heat and entirely bum ; 

 it will, however, take a stronger 

 heat to destroy the particles of 

 animal matter of the oyster — the 

 young spawn which will naturally 

 be brushed into the water. This 

 oyster-shell process will give Pleu- 

 rosigmas and Coscinodiscus in abun- 

 dance. The fresh-water diatoma- 

 ceae from the Citronelle deposit 

 contains more than thirty kinds 

 of diatoms, many of which will come 

 in the field of view at one time ; and 

 for variety, surpasses any of the 

 fossil deposits of nearly twenty 

 localities that I have examined. 



K. M. Cunningham. 



Micrometry and Collar- Adjust- 

 ment. 



In his deservedly popular book 

 llmv to Work with the Microscope, 

 Dr. Beale insists, with good reason, 

 on the importance of making draw- 



ings of objects examined, and points 

 out that the value of such sketches 

 is often greatly diminished by the 

 absence of any statement of the 

 magnification used in making them. 

 But he has fallen into an error 

 which should be corrected, because it 

 is made by an acknowledged expert. 



He recommends that scales be 

 drawn or printed, showing the size 

 to which hundredths or thousandths 

 of the inch or centimeter are mag- 

 nified by each of the objectives 

 used ; and one of these scales cor- 

 responding to the objective em- 

 ployed, he advises, should be pasted 

 on every drawing. He has omitted 

 to mention the fact that in all ob- 

 jectives made with a collar-adjust- 

 ment, the magnification at the 

 " open " and " closed " points 

 varies so much, that scales made in 

 the manner suggested would be not 

 only worthless, but misleading. 



The fact of a difference existing 

 is, of course, well known, and 

 among the methods of nomencla- 

 ture for objectives, one having 

 strong support is based on the mag- 

 nification without ocular and at the 

 " closed " point, with a ten-inch 

 tube. The amount of this differ- 

 ence, however, does not seem to 

 have been sufficiently taken into 

 account. This will be best illustrated 

 by a table showing the variations 

 in a few objectives of well-known 

 makers, taken with a tube ten 

 inches in length, measured from 

 the stage-micrometer to the end of 

 the tube proper (not to the end of 

 the eye-piece) : 



Objective. Oculars. 



A. B. C. 



Geo. Wale i-6 inch, open 262 433 680 



do closed 283 466 725 



Powell & Lealand 1-8 inch, open.. 392 650 1025 



do closed 500 833 1300 



Spencer & Sons i-io inch, open. . . 462 750 1200 



do closed. . 533 887 1400 



Wm. Wales i-is inch, open 517 850 1350 



do closed 733 1200 1900 



It will be seen that the range in 

 magnification is greater with some 



