and Laboratory Methods. 



1441 



higher magnification and with oblique illumination to show details, as in Figs. 

 1, 2 and 3. 



The exposure is made as short as possible without sacrificing detail; then, if 

 the plate be strongly developed the requisite contrast will be secured. I find, 

 too, that it is best to make two exposures of each specimen. 



-On removing the plate from the holder a number is placed on one corner with 

 a soft lead pencil, say Dixon's " Ultimatum "' or one 

 similar to it. This number is also placed in a book 

 kept for the purpose with the name of the species, 

 date, magnification, light, number of slide, and loca- 

 tion. With these data it is an easy matter to re- 

 photograph any particular specimen if at any time 

 the negative be lost or broken, or if for any reason 

 it prove unsatisfactory. 



I have experimented with every developer to be 

 had here, and have tested many formulas, but none 

 of them is equal to the one known as Bromo- 

 hydroquinon. It gives the requisite amount of con- 

 trast, a thing to be kept constantly in mind in 

 photographing objects so very hyaline as are diatoms. 



For a fixing bath plain hypo seems to give better 

 results than the acid alum bath. 



Any good plate will answer the purpose provid- 

 ing it is heavily coated ; my preference, however, is 

 " Cramer's Instantaneous Isochromatic." Let me 

 say again, a thin plate will not answer. In order 

 to economize, I get the 4x5 plates and then cut 

 them once or twice as the size of the diatom de- 

 mands ; i. e., the plates are then 2^ x 2, or 2;^ x 4. 

 When dry they are put in appropriate envelopes, 

 filed away in alphabetical order, and a full record of 

 each one is kept in a card index. 



Most diatoms lend themselves readily to photography, the side of the valve 

 which is most important usually being nearly plane. Some, however, are more 

 or less convex or concave. Figure 5 represents a species that has a ridge just 

 inside the margin and a depressed center. This of course necessitates a com- 

 promise, with some loss of detail. Only a very few species of the fresh water 

 forms, however, are impossible of photography as here described. 



As is well known, Navicula is the typical genus of the Bacillariaceae, with 

 hundreds of species ; these come out beautifully, as is attested by Fig. 4. 



Previous to printing, the negative is placed in a retouching frame and the 

 background is all cut out by the application on the back of a heavy coat of 

 " Copelin's Opaque." This cuts out everything but the image desired. For 

 printing, all sorts of paper have been tried. Among those that I have used, of 

 the developing sort, Velox, and of the printing sort, Solio, seem to give the best 

 results. I have discarded Solio, however, on account of its slowness. In print- 



Fig. 3. 



