180 THE AMERICAN MONTHLY [August, 



Mueller, Die Rostpilze der Rosa und Rubusarten. Inaug. Diss., 18S6. 



Parker, On the morphology of Ravenelia glandulseformis. Pi'oc. 

 Amer. Acad. Sci., xxii, 1886, pp. 205-219. 



Plowright, Heteroecism. Gard. C/iron., xix, p. 824 ; British Ure- 

 dineas and Ustilagineas, 1S89. 



Saccardo, Sylloge Fungorum, vii, 1888. 



Sorauer, Pflanzenkrankheiten, ii, 2d ed., 1S86. 



Von Thuemen, Melampsora salicina, der Weidenrost, 1879. 



Ward, On the Sexuality of the Fungi. £Htart. Jotir. Micr. Set., 

 xxiv, n. ser., pp. 262-310. Life History of Hemileia vastatrix. Jour. 

 Linn. Soc, xix, 1882, pp. 299-335. 



Winter, Rabenhorst's Kyrptogamen Flora, Pilze, i, 1881-7, pp. 

 131-270. 



Purdue University, June i, 1889. 



Magnification in Photo-micrographs. 



By W. J. SIMMONS, 



CALCUTTA, INDIA. 



A friend of mine, Mr. Walter Osmond, who photographs a good deal 

 with the microscope, inquires how he should fix the magnification of 

 his objects, as shown in his pictures. Using a ^-inch objective and an 

 ocular, which together give 300 diameters, with the eye-glass at 10 

 inches from the paper, he gets a field as nearly as possible 6 inches in 

 diameter. Employing the same eye-piece and objective in his camera, 

 he gets a disc in his photos at 2.9 inches. He fixes the magnification 

 in the photo at 145 diameters by the following simple rule-of-three 

 sum : 6:2*9:: 300: 145. Similarly, with a 2-inch objective and a pow- 

 erful ocular, which together give 55 diameters at the standard height 

 of 10 inches, he fixes the magnification of his photos at 26^, thus : 

 6 : 2 • 9 : : 55 : 26 • 583. Is this correct ? You will observe I distinguish 

 in this note between the "magnifying power" of an objective and 

 "magnification," implying by the latter term mere enlargement ; and 

 confining the former " magnifying power" to that particular degree of 

 enlargement which is obtained when an image is projected by any kind 

 of microscopic camera, on a plane horizontal surface parallel to the 

 body of the microscope, and distant exactly 10 inches from the centre 

 of the eye-glass of the ocular. I would also add that such rough 

 measurement as can be made by opening both eyes and comparing an 

 object with a rule laid at the stage serves to show that Mr. Osmond's 

 method fs correct. 



An Enterprising Druggist. — Mr. G. A. Waltenspoil, of Jackson, 

 Cal., keeps a microscope in the store for examining drugs. Frequently, 

 when he mounts an interesting object, he places the microscope on the 

 counter for anyone who comes in to examine. This practice tends to 

 attract customers, turns attention to his store, and educates the public. 

 He commenced with a lense magnifying only twenty-five diameters, but 

 now uses one hundred and fifty diameters, and frequently makes exami- 

 nations for physicians. 



