and Laboratory Methods. 



1439 



LABORATORY PHOTOGRAPHY. 



Devoted to methods and apparatus for converting an object into an illustration. 

 PHOTOGRAPHING DIATOMS. 



In photographing diatoms at the University of Iowa the apparatus used is of 

 the simplest character. It consists primarily of a " Practical " photomicro- 

 graphic camera, a microscope furnished with a mechanical stage, apochromatic 

 lenses, and with compensating and projection eyepieces. The remainder of the 

 apparatus is mostly home made and consists of a table, condensing lenses suit- 

 ably disposed, an acetylene generator, and simple lamp or burner. 



The camera is hinged so that it may be used in either a vertical or horizontal 

 position. I find this very convenient, as the bellows may be quickly raised to 

 allow the operator to make a direct examination of the object. The working 

 lens is a dry apochromatic, 3mm. of .95 N. A. A compensating eyepiece No. S 

 and a projection eyepiece No. 4 are the oculars used. The table, 

 which serves the purpose at once of camera table and optical 

 bench, is about three feet six inches long. The width is about 

 sixteen inches and the height so adjusted that when the operator 

 sits in a chair the ocular is in a convenient position for obser- 

 vation. 



When making an exposure the bellows is turned down and 

 rests on the leaf of the table, which for this purpose is raised to a 

 horizontal position. 



The condenser is composed of two plano-convex lenses two 

 and one-half inches in diameter, an achromatic pair two and one- 

 fourth inches in diameter, and a one-inch negative to effect the 

 parallelism of the rays. The spherical and chromatic aberration 

 of the first system of condensers is in a large measure corrected 

 by this simple device ; and, although it is conceded that every 

 additional lens is in a sense an added obstacle, nevertheless the 

 advantage to be derived from the introduction of the negative in 

 the series at this point will quickly become apparent to anyone who chooses to 

 try the experiment. 



No heat filter is necessary with acetylene gas as the illuminant. As is well 

 known, this light is remarkably cool. The substage condenser is a plain, uncor- 

 rected Abbe. 



A small acetylene generator and a round flame burner complete the outfit. 

 I have adopted the round flame burner after a series of experiments involving 

 every other form of burner offered by the trade. I certainly consider it the most 

 desirable and efficient. 



In the present discussion I shall assume that the material to be photographed 

 is properly mounted in well cleaned styrax on cover-glasses of known thickness. 

 For mounting the larger species a mechanical finger will be found convenient, as 

 such species should be mounted singly. In photographing from spreads I pro- 



Fig. 



