2G4 ROOD, ON THE PRACTICAL APPLICATION OF 



was found to answer very well, both as to the degree and 

 quality of the light, negatives being obtained in fifteen seconds, 

 enlarged 1500 diameters. As the chromatic aberration was 

 not corrected, it was found easy to illuminate the object either 

 with white or bluish-white light, use of the red or yellow rays 

 being, of course, carefully avoided. 



The proper distance of the condenser from the object is a 

 point of much importance, and is best ascertained by carefully 

 repeated trials. To, obtain really good results, much nicety 

 in arranging the illumination is required ; this is a matter 

 in which microscopists are well practised, but to secure the 

 best results possible under the circumstances, as in photograph- 

 ing test-objects, the art and patience of the operator is taxed 

 to the utmost, and several days are often consumed before a 

 really satisfactory result is attained, even in the case of a 

 single object. 



Focal adjustment, ^c. — Trouble will be saved by selecting 

 the exact object which is to be photographed by the micro- 

 scope in an upright position ; the instrument is then inclined 

 and connected with the camera. After the compound body 

 is thus placed, if the objective is provided with a "screw 

 collar ^^ for correction, this adjustment must be carefully made. 

 Even Avhen this point has received attention, it by no means 

 follows that the chemical focus coincides with the ^^sual, and 

 the exact correction necessitated by this difference must be 

 ascertained by trial. This can be effected by the use of the 

 fine adjustment (see Shadbolt's paper). Contrary to some 

 other observers, I have found it necessary when using sun- 

 light, with both high and low powers, with and without eye- 

 pieces, to make this correction carefully. The use of the rod 

 and the lever and achromatic lens has already been mentioned. 

 After the corrected image has been thrown on the ground 

 glass, it will remain nearly unaltered from thirty seconds to 

 ten minutes, according to the power and mode of illumination 

 employed. 



Collodion. — This article, when furnished bymakers of repute, 

 can, of course, be used, though it is better, for more than one 

 reason, to be independent of the dealers, if possible. A con- 

 siderable number of samples of pyroxyline were prepared 

 according to different receipts, and sensitized variously. The 

 very simple process described by Waldack, on page 266 of 

 his ' Treatise on Photography,' was found, with slight modifica- 

 tions, to yield an excellent article."^ The strength of the 

 sulphuric acid was slightly greater than recommended by 

 him, no water was added, the temperature also was slightly 

 * ' Treatise on Photography,' by Chas. Waldack, Cinciunati, 1860. 



