214 PRACTICAL MICROSCOPY. 



emulsion. Mawson and Swan's, having been extensively 

 used by the author for this kind of work, deserve commen- 

 dation as being of good and uniform quality. 



Do not over-expose these gelatine plates, or you will get 

 nothing but thin and misty images. From some cause, 

 those exposed to gas or lamp-light do not commence to 

 develop so quickly as those exposed to sunlight ; but if 

 time is given, and they are not over-exposed, perfect 

 pictures may easily be obtained. The time of exposure in 

 the camera depends upon the quality of the light, but more 

 perhaps ttpon the nature of the object than anything else ; for 

 instance, a section of the kidney of a horse required twenty 

 minutes' exposure with the argand gas and bull's-eye con- 

 denser, while a crystal of glass under the same conditions 

 of light and amplification required but one minute. 



For ascertaining the exact exposures and the corrections 

 required for the difference in actinic and visual foci, the 

 beginner is strongly advised to keep a bound record of his 

 failures as well as prints from all his negatives, with full 

 particulars respecting them ; they are great helps in photo- 

 graphing new subjects. 



Mr. Dancer of Manchester, Dr. Maddox, Mr. Shadbolt, 

 the late Dr. Redmayne of Bolton, Mr. Wenham, and 

 several others have been the most successful photographers 

 of microscopic objects ; not forgetting, however, Colonel 

 Dr. Woodward in America, as most of the photographs 

 and nearly the whole of the literature of this country has 

 emanated from him. 



Dr. Woodward has described his method with sunlight 

 and a heliostat, from which we gather the following : 



The microscope being placed on a shelf at the window of 

 the dark room, and its body made horizontal, the achro- 

 matic condenser is illuminated by a solar pencil reflected 

 from a heliostat (Fig. 196) upon a movable mirror outside 



