Journal of Applied Microscopy. 



115 



The Construction of Achromatic Doublets 

 and Triplets. 



E. M. Nelson, in his presidential 

 address to the Royal Microscopical 

 Society (Journal Royal Microscopical 

 Society, 1898, part 2, pp. 156-169), 

 explained, with several examples, the 

 methods of calculating achromatic 

 doublets and triplets, including the kind 

 of glass to be used, and the radii and 

 thickness of the elements. The paper is 

 of the utmost interest to microscopists, 

 and Mr. Nelson, by his admirable sys- 

 tem of illustration by the numerous 

 examples, has made it comprehensible 

 to even those who have no great knowl- 

 edge of mathematics, and all must hope 

 for an extension of the subject to the 

 construction of apochromatics. 



P. E. B. JOUKDAIN. 



Derbyshire, Eng. 



Diagnosis and Description of Bacteria.* 



In order to secure uniformity in the 

 description and differentiation of species 

 of bacteria a committee of American 

 bacteriologists, consisting of J. George 

 Adami, A. C. Abbott, T. M. Cheesman, 

 George W. Fuller, W. T. Sedgick, Charles 

 Smart, Theobold Smith and AV. H. 

 Welch was appointed. In the preface. 

 Dr. Smart gives the history in connec- 

 tion with the movement of the American 

 Public Health Association to adopt some 

 uniform methods in the description and 

 differentiation of species of bacteria. 

 The paper should be in the hand of every 

 bacteriologist. It would be well to re- 

 print the pamphlet and put it in book 

 form. The pamphlet includes the stand- 

 ard charts for bacterial diagnosis com- 

 piled by Dr. Cheesman. These are most 

 conveniently arranged and add not a 

 little to the value of the pamphlet. 



Ij. H, Pammel. 



♦Procedures Recommended for the Study 

 of Bacteria, with Special Reference to 

 Greater Uniformity in the Description and 

 Differentiation of Species. Rep. Comm. of 

 Am. Bacteriologists to the comm. on the 

 pollution of water supplies, of the Am. 

 Public Health Association, Philadelphia 

 meeting, 1897; separate. Concord, Mass., 47; 

 5 charts. 1898. 



The Centration of the Electric Arc in 

 Photo-Micrography. 



J. E. Barnard and T. A. B. Carver 

 (Journal Royal Microscopical Society, 

 1898, part 2, pp. 170-173), found that, with 

 an automatic lamp, fluctuations in the 

 intensity of the light and diffraction 

 phenomena and shadows were unavoid- 

 able. That the latter were due to decen- 

 tration of the light was proved by 

 projection of the image onto a lined 



screen, by means of which movement of 

 the image could be observed. These 

 defects of the automatic lamp, together 

 with the fact that inconstancy of the 

 length of the arc is, a priori, unavoidable 

 in that form, led Messrs. Barnard and 

 Carver to design a hand-feed lamp in 

 which the relative positions of the car- 

 bons, held in V-clamps which slide upon 

 two rigid vertical rods, are varied by 

 a right and left-handed screw, and the 

 position of the pair together by another 

 screw. The indicator of centrality con- 

 sists of a small, cylindrical, metal 

 pin-hole camera, mounted on a universal 

 joint, so that an image of the arc is 

 cast upon a ground glass screen provided 

 with reference lines. The question of 

 inclination of the carbons to the vertical 

 was investigated by means of an appa- 

 ratus giving revolution of the arc about 

 a horizontal axis passing through itself; 

 and the "critical inclination"— the great- 

 est at which none of the light from the 

 crater is cut off by the negative carbon 

 —was found to be about twenty-seven 

 degrees. 



P. E. B. JOURDAIN. 

 Derbyshire, Eng. 



Collection of Pond Life on Photographic 

 Plates.* 



While examining some ordinary pro- 

 tographic plates under the microscope, 

 Mr. Wallace Goold Levison, S. B., found 

 an interesting way of catching and hold- 

 ing minute organisms for >ixamination 

 under the microscope. He found num- 

 bers of these forms adhering to gelatin 

 coated photographic plates after the 

 plates had been in the water the usual 

 time allowed for washing them after 

 coming out of the hypo solution. The 

 number of forms varied with the length 

 of time the plates were in the water. By 

 placing the plates in a box used for 

 washing the hypo from the plates and 

 letting the city water run through it, 

 he collected on them a few large dia- 

 toms, many smaller ones, and a large 

 number of small active forms. When 

 the film had become soft after being in 

 the water several days, vorticellae and 

 other infusoria appeared anchored to 

 the film. These forms appeared al- 

 though this box was in a dark place and 

 was frequently used for washing the 

 hyposulphite solution from the plates, 

 and for washing acid solutions from 

 metal plates. For convenience in ex- 

 amination, he cut the plates into small 

 pieces, so that he could put them in a 

 cell, film side up, and examine them with 

 a thin layer of water over them. His ex- 



- *Preliminary note read before the New 

 York Microscopical Society. 



