P1iotogra])hing Histological Preparations hij Sunlight. 173 



chromatic, is corrected mainly by tlie just combination of flint with 

 crown glass in the several pairs which constitute the objective. If 

 these are so adjusted as to correct spherical aberration as nearly as 

 possible for white Ught, they will no longer do so for light which 

 has passed through the ammonio-sulphate of copper. Until objec- 

 tive makers take this fairly into consideration the microscopist who 

 desires to photograph what he sees is left to a happy chance in the 

 selection of his objectives. For even those makers who profess to 

 prepare objectives " specially corrected for photography " do not 

 deal any too well with the problem. If they would test their 

 objectives, while making them, with violet light, we should have 

 better results ; for with such illumination the eye can see all that 

 photography can execute and no more. 



But this circumstance fortunately enables the microscopist to 

 select from the objectives in the market those which are suitable for 

 photography. It is only necessary to test their performance when 

 illuminated by sunlight which has passed through an ammonio- 

 sulphate cell. Now it fortunately happens that the high-power 

 immersion objectives of certain makers, especially those of Powell 

 and Lealand, possess very nearly the corrections which theory would 

 indicate as best adapted for photographic use. Nevertheless it can 

 hardly be doubted that even these objectives could be greatly 

 improved if the makers would take into consideration the principles 

 involved in the foregoing remarks. 



A second point, which deserves attention, is the use of the large 

 condensing lens above described. This lens, it will be understood, 

 corresponds with the large condensing lens of the ordinary solar 

 microscope, while the achromatic condenser takes the place of the 

 so-called field-glass of the same instrument. It has already been 

 mentioned that this lens should be placed at such a distance fi'om 

 the achromatic condenser that the solar rays may be brought to a 

 focus, and begin again to diverge before they reach its lowest glass. 

 A diflerent arrangement is usually employed in the solar microscope, 

 the field-glass being placed at such a distance from the first 

 condenser that the solar rays impinge upon it before they come to 

 a focus. As a consequence, the convergent pencils proceeding from 

 the first lens are still further converged by the field-glass, and a 

 burning focus of heat, as well as of light, is produced, which is 

 damaging to the preparation as well as to the balsam cement of the 

 objectives used. If, however, the rays from the fiirst lens are per- 

 mitted to come to a focus and to begin to diverge before striking 

 the second, this latter can readily be adjusted so as to bring the 

 illuminating rays to a handsome focus, while the heat rays, after 

 passing the second lens, become parallel or even divergent according 

 to the position of the achromatic condenser, and all trouble from the 

 solar heat is thus completely avoided. So successfully may this 



