and Laboratory Methods. 1199 



LABORATORY PHOTOGRAPHY. 



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

 THE PROCESSES OF PHOTO-MICROGRAPHY. 



In a previous paper, I described the apparatus employed in high-power photo- 

 micrography and outlined the method used for the projection of the luminous 

 image upon the focusing screen. To complete this description and to reduce the 

 whole matter to a concrete example, I will here explain the further steps involved 

 in the production of a negative, giving actual details of exposure time, speed of 

 plate, etc. 



Before proceeding to a discussion of processes, however, it will be well to 

 consider the materials which are to be employed. Chief among these is the sen- 

 sitive plate designed to receive the light impressions and to make permanent 

 record of them. Formerly the old collodion wet plate was used for this purpose, 

 and many of the best photo-micrographs ever made are the productions of the 

 earlier workers who employed this process. Workers now-a-days, though, do not 

 resort to the tedious methods the use of the wet plate involves, but choose the 

 more convenient and rapid gelatin dry plate. 



Of dry plates, there are any number on the market, and most of them are 

 good for ordinary photography ; but in photo-micrographic work the conditions 

 are different from those which prevail where the image is smaller than the object 

 and the light is plentiful. The strong diffusion of light involved in the produc- 

 tion of an image 1000 diameters larger than the object itself renders it expedient 

 to use plates that will record strongly all dififerences of lighting and thereby pro- 

 duce negatives with good "contrast." In this respect the slower plates, those 

 of lower sensitometer numbers, are the best ; and if the operator has any one 

 with which he is familiar, he will do well to make use of it when undertaking the 

 unfamiliar work of photo-micrography. 



To such as are not adept in the manipulation of any particular plate, I would 

 strongly recommend the orthochromatic " Carbutt Process Plate." After an 

 extended trial of many plates, I find this one in a large degree satisfactory. It 

 produces negatives with clear, sharp details and abundant contrast. The 

 film is hard and firm, washes readily, and dries quickly. Beginners will make no 

 mistake, I am sure, in starting with this plate. Since so many sections are 

 stained by some one or other of the blue dyes, it is best to make use of the ortho- 

 chromatic plates, as they give better balanced negatives. The " Process " plate 

 may be obtained in this form and I prefer it to the plain form. 



Next in importance to the plate is the developer with which the latent image 

 upon it is made manifest. Here, as in the former case, I have nothing to rec- 

 ommend to those who are well acquainted with the action of any good develop- 

 ing fluid. The agent itself is really not of so much importance as is the knowl- 

 edge of how to use it. By this, I do not mean that there is no choice, but it is 

 better to manipulate a poor developer well than a good one poorly. 



It is recognized that certain reducing agents, e. g., eikonogen, produce " thin " 



