PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 795 



semicircular scale mounted on the microscope stand. This scale has a slider with 

 four 4° divisions. Each 1° is made to equal }/i n change in focus. The first division 

 of the slider is set to the pointer at the starting position. The slider is a convenient 

 means to show the starting position and the number of exposures in the photographic^ 

 sequence. This arrangement is shown in Fig. 21. A modification of the same idea, 

 provides an extension of the slow-motion drum which terminates in a larger drum 

 divided into microns and these subdivided into quarters. 



Fig. 25. — Living sperm cells of the grasshopper. (Magnification 500 X.) 



Photographic Materials for Ultraviolet Microscopy. — The most suitable plates for use 

 at 2750 and 2573 A. are the process-type plates. Plates which can be recommended 

 are the Eastman Process, Eastman Contrast Process, and Hammer Process. Used 

 with a contrast developer such as the Eastman D-IQ,^ they give excellent results. 

 When absorption in the specimen is not very strong, the Eastman Contrast plates are 

 recommended. For the wavelengths between 2500 and 1900 A. special ultraviolet- 

 sensitive plates are required as the ordinary plates are insensitive at these wavelengths. 



1 Eastman D-19 Contrast Developer for Metallographic Photography 



Stock Solution 



Water (about 125°F.) 64 oz. 2.0 1. 



Metal (Elon) 128 gr. 8.8 g. 



Sodium sulphite (desiccated) 12 oz. 360 gr. 384.0 g 



Hydroquinone 1 oz. 75 gr. 35 . 2 g. 



Sodium carbonate (desiccated) 6 oz. 180 gr. 192.0 g. 



Potassium bromide 300 gr. 20 . g. 



Cold water to make 1 gal. 4.01. 



Use without dilution. Develops 3 to 6 min. in a tray at eS^F. according to the contrast desired. 



