San Francisco Microscopical Society. 275 



SAN FRANCISCO MICROSCOPICAL SOCIETY. 



[We are indebted to Mr. William E. Lo^^ the able recording 

 secretar\\ for the following account of an instructive exhibition 

 given by this society August 19, 1891.— Editor.] 



A. H. Breckenfeld showed a beautiful live water spider, order 

 Arachnidaj, genus Hydrachnea, on a dark field, which attracted a 

 great deal of attention. Henry C. Hyde gave a pleasing exhibit of 

 diatoms, one slide showing one hundred different species of navi- 

 culae, mounted in sty rax. S. E. Taylor showed a single frustule of 

 a species of Arachnoidiscus. a beautiful genus of diatoms, with a 

 low-angled half-inch objective and dark held illumination. L. M. 

 King had on his stand an arranged slide of diatoms prepared by 

 Rinnbock of Vienna. Charles C. Riedy exhibited a Navicula lyra 

 with a one-fourth inch apochromatic objective and dark field. 



R. H. Freund showed the onlv ])roi)arati()n of bacteria on exhi- 

 bition, a species of pathogenic l^acteria not identified. E.S. Runyon 

 had a number of very attractive crystals, including the cr\^stalizable 

 princi])le from the prickly pear, Platino-cyanide of magnesium and 

 Mcnthon crvstals, all shown with polarized light and excellent 

 effect. William E. Loy sh<}\ved a young star-fish, Astcrium gibbosa, 

 with the aid of a spot lens. 



The exhibition was further enhanced by brief informal ad- 

 dresses. Mr. Hyde gave an outline history of the Diatomacee, show- 

 ing first t;heir place in botany, how generalh' the}' are dispersed 

 about the globe, where they may ht found, and a comparison of the 

 fossil and recent forms. The peculiar structure of the diatom has 

 preserved very perfect skeletons from the earliest geologic eras in 

 which it is found, and when properly treated these fossils ma}' be 

 cleaned and mounted, yielding the most beautiful of all microscopic 

 objects. He remarked that since the time of Ehrcnberg many scien- 

 tists had spent years of patient study in this captivating field, so 

 that to-day many thousand species had been figured and described. 

 Unlike some objects in nature, new beauties are seen with increased 

 amplification, and the highest powers of the microscope only serve 

 to increase the admiration of the beholder. 



A. H. Breckenfeld gave a brief demonstration of the optical 

 qualities of what is known as dark-field illumination. By this 

 method the light is thrown around and upon the object, but its 

 ravs do not enter the objective nor reach the eye of the observer. 

 The object then is shown on a dark back-ground, with a brilliancy 

 not possible where the light passes directly through the object. His 

 remarks were fully illustrated by drawings on the blackboard, and 

 proved instructive and interesting. William E. Loy. 



