78 



THE AMERICAN MONTHLY 



[April, 



NOTES. 



— Messrs. Emmerich & Son have re- 

 ceived copies of the EngUsh catalogue of 

 Zeiss, which they will send to any address 

 for ten cents, as announced in their ad- 

 vertisement. We have no doubt this new 

 catalogue will attract much attention. 

 Mr. Zeiss is so well known among the 

 microscopists of this country that a de- 

 scription of his stands and apparatus in 

 English will be of great interest. 



— Mr. W. G. Blish, in the Scientific 

 Americaii, states, that to preserve paste 

 eels, the paste should be kept in a wide 

 mouth bottle, loosely stoppered, placed in 

 a cool place. If the eels are not doing 

 well, he adds a piece of bread, or prepares 

 some fresh paste, preferably of rye flour. 

 Paste containing a good supply of eels 

 will keep for weeks without moulding. 



CORRESPONDENCE. 



Magnification. 



To THE Editor : — Sufficient time has 

 been given for answers to the series of 

 questions propounded on page 240 

 (vol. vi), in reference to magnification. 

 Without wishing to be at all personal, the 

 cjuestions evidently were not understood, 

 or no measurements were made. Were 

 it not for the signature, the so-called an- 

 swers on page 20 of the current volume 

 would not be worthy of serious consider- 

 ation. It would be well to have more 

 practical measurements and less theory. 

 It is a ' poor rule that does not work both 

 ways.' In answering the first question 

 he says a one-inch lens magnifies 7.9 di- 

 ameters, and on page 1 1 of the catalogue 

 which he refers to a one-inch eye-piece 

 magnifies 11 diameters, and on page 12 

 a one-inch objective magnifies g diam- 

 eters. 



I am pleased to know that a lo-inch 

 tube is just 10 inches long (254 mm.), 

 but after 30 years' experience I have 

 never seen a microscope with a tube of 

 that length — a full length tube of a ma- 

 jority of microscopes is about 9 inches. 



I am acquainted with the different the- 

 ories in reference to it, but would it not be 

 better to define what is meant by giving 

 the distance from the front of the objec- 

 tive to the top of the eye-piece, or from 

 the front of the objective to the diaphragm 

 of eye-piece ? Not one person in a hun- 

 dred can locate the posterior focus of his 



objective and measure the distance from 

 the diaphragm of the eye-piece. 



In regard to the first question, the near- 

 est lens which I have is -^^^ of inch fo- 

 fcus, and it magnifies 9.23 times ; a i-inch 

 lens would magnify about 9.2 diameters. 

 The focus was found by measurement 

 with a focometer. The theory on which 

 it is constructed is that the distance be- 

 tween conjugate foci of a lens is just four 

 times the focus for parallel rays. 



In regard to the second question, Mr. 

 ToUes' formulas for a two-inch eye-piece 

 have been given as follows : — 



1. Field glass, radius 1.5, aperture 1.2. 

 Eye glass, radius 0.8, aperture 0.59. 

 Distance apart 2.6, flat sides. 



2. Field glass, radius 1.4, aperture 1.12. 

 Eyeglass, radius 0.8, aperture 0.59. 

 Distance apart 2.5. 



3. Field glass, radius 1.3, aperture 1.12. 

 Eye glass, radius 0.79, aperture 0.59. 

 Distance apart 2.42. 



In reply to the third c|uestion, I would 

 say the nearest lens I have has a focus of 

 2.05, and magnifies 3,873 times. The 

 estimated magnifying power of a 2-inch 

 lens is about 4.5 diameters. The first 

 formula for a 2-inch eye-piece I have not 

 measured, but I believe it would magnify 

 about 4.25 diameters. The second form- 

 ula gives magnification of 4.5, and the 

 third 4,348 diameters. 



The fourth and fifth c[uestions are still 

 open. 



Walter H. Bulloch. 



Chicago, 111. 



Durability of White Zinc Cement. 



To THE Editor : — Some two years ago 

 I bought Mr. A. C. Cole's Series No. 3 

 Educational Preparations, including 24 

 slides. The slides are perfect models of 

 neat mounting, cell rings of white zinc. 

 In no case has the cement run in. I 

 found the other night that a slide of adi- 

 pose tissue had begun to spoil. On hold- 

 ing the same to the light, 1 found on ex- 

 amining the ring with a small lens a great 

 number of transverse cracks, caused by 

 shrinkage. I have turned a fresh ring of 

 marine glue in fusil oil overlapping the 

 white zinc on all the slides. 



What would you suggest ? 



Fr. Dienelt. 



[White zinc cement, when it hardens so 

 much that it cannot run in, is very likely 

 to crack after a while. The best remedy 

 is a coat of shellac in alcohol, and a fresh 

 ring of white zinc cement outside of that 

 if the color is objectionable. The prep- 

 aration is then perfectly secure. — Ed.] 



