CHAPTER VI. 



EXPOSURE DISCOLOURED BALSAM AN EXPOSURE 

 RECORD EXAMPLES OF EXPOSURE. 



So much depends upon the light, the power of 

 lens and eyepiece used, the colour and opacity of 

 the object, camera extensions, etc., that only an 

 approximate estimate may be made from the re- 

 sults of another worker ; but the exposures tabulated 

 hereafter were obtained with a paraffin lamp having 

 a wick one inch wide. A word of warning may be 

 given concerning specimens mounted in balsam. If 

 one of these show patches of deeper colour in some 

 parts than others, or if the gold size, marine glue, 

 or other material of which the cell is constructed 

 has dissolved and stained its contents, it should be 

 rejected forthwith, for no amount of tinkering will give 

 a good negative. Slides deep yellow on one side and 

 perfectly transparent on the other, whilst not unlit 

 for visual examination, are quite unsuitable for 

 photography. This shows that some attention 

 ought to be paid to the quality of the mounting. 



A record of all exposures should be kept, entering 

 the failures as well as the successes ; it should note 

 the plate used, the class of object photographed 

 with its characteristic quality, the exposure, the dis- 

 tance of the plate from the object, and the result 

 with each power and with each class of illumination. 

 Supposing this to be done, and one wishes to< know 



