140 BACTERIA IN RELATION TO PLANT DISEASES. 



is desired. When using oil-immersion objectives see that there are no air-bubbles 

 or particles of dirt in the cedar oil. The image on the ground glass should be 

 observed the last thing before introducing the plate-holder, to see that it is free from 

 images of objects not actually embedded in the slide. For the same reason slides 

 and covers for mounting objects to be photographed must be cleaned with great 

 care and kept clean until ready for use. Many really beautiful sections are ruined 

 for photomicrographic purposes by having been mounted in dirty balsam or 011 

 dusty slides, or by being covered with soiled cover-slips. Sections should be cut and 

 mounted in dust-free air, and the balsam used in mounting must be free from dirt. 

 Much balsam on the market is very dirty and totally unfit for mounting sections 

 designed to be photographed. 



Glass surfaces through which it is designed to pass light should not be touched 

 by the hands, greasy or otherwise. This applies to slides, covers, objectives, con- 

 densers, ray-filters, photographic lenses, mirror surfaces, ground glasses, negatives, 

 lantern-slides, and what not. The least touch of the finger on a polished glass 

 surface generally leaves its mark. 



In using the common achromatic objectives for making photomicrographs, the 

 "focus-difference" must be taken into account. Such objectives, being corrected 

 only for two portions of the spectrum, require a different focus for the sensitive 

 plate than for the human eye. In other words, au image which is perfectly sharp 

 to the eye is not sharp for the sensitive plate and will yield a negative which is 

 out of focus. By turning the fine adjustment a measured distance the image 

 becomes hazy on the ground glass, but will then yield a sharp negative. A few 

 exposures will determine just how much and in which direction the eye focus must 

 be thrown out to give the sharpest result. The focus-difference may also be disposed 

 of by using monochromatic light. The writer uses such light almost altogether, 

 even with the best objectives. Another defect of achromatic objectives, and to some 

 extent of all objectives, is an arching field, the center being out when the edges 

 are in sharp focus. For this reason it is customary to select a small portion in the 

 center of the field, make this as sharp as possible, and neglect the margins, which 

 may be trimmed off on the print. The Spencer i6-millimeter apochromatic objec- 

 tive has the flattest field of any objective of like quality known to the writer. Lack 

 of depth of focus is a serious defect in photomicrographic work, and must be com- 

 pensated for by making the sections uniformly thin and mounting them perfectly 

 flat. The student should read Steruberg in English and Neuhauss in German 

 (Bibliog., LV). 



For most stained sections involving bacteria, isochromatic plates are to be 

 preferred, and slow rather than rapid ones. Exposure should be for contrast, and 

 consequently as short as will give the necessary detail in the heavily stained parts. 

 Development should be rather long and with an effort to obtain good contrasts. 

 The writer formerly used hydrochiuou, but now uses pyro, and develops until the 

 image is visible on the back. 



For general photographic work ortol is an excellent developer, and its prepara- 

 tion is extremely simple. If one uses the Hauff mixtures sold by Gennert, of New 



