114 THE MICROSCOPE. 



into the combination. By this means it is possible to entirely 

 remove the secondary spectrum, leaving only a residuum of a 

 tertiary character, which is got rid of by specially constructed 

 eye-pieces, called compensating eye-pieces, the result being that 

 images perfectly free from colour are obtained. One of the 

 many advantages which these lenses possess is that the power 

 can be increased by specially constructed eye-pieces to a much 

 greater extent than is possible with the old lenses. By this 

 means one objective can be made to do the duty of several ; and 

 the microscopist of the future will only require about three ob- 

 jectives to do everything he wants. 



Through the courtesy of Dr. Anthony, I had an opportunity 

 of testing the performance of two of these objectives, and com- 

 paring them with lenses of ordinary make but of very fine qual- 

 ity, and the result was very much in favor of the new form. 

 Compensating eye-pieces were used with all the objectives tested 

 as they give better results, even with objectives of the ordinary 

 construction, than the Huyghenian form generally used. The 

 apochromatics were tested on a variety of objects with the deep- 

 est eye-pieces, and there was not the slightest indication of their 

 breaking down. To get a better idea of the superiority of these 

 lenses, the objects were first of all viewed with some objectives 

 of the old construction, but of very fine quality. These were 

 tried with the highest eye pieces they would bear, and were 

 very satisfactory, one very fine quarter-inch running the apo- 

 chromatic of about the same power pretty close ; but there was 

 no mistaking the superiority of the latter under the deepesteye- 

 pieces, all the minutest details coining out perfectly sharp and 

 clear to the last. Of course, the illumination was of the best, 

 a prism being used instead of the plain mirror, and the first- 

 class condenser was most carefully adjusted before commencing 

 the trial, the performance of an objective being very much im- 

 proved in proportion as these details are attended to. The ob- 

 jectives referred to were dry lenses, and in power equivalent to 

 an English inch and quarter- inch. Another advantage of these 

 lenses is the large working distance, and the facility with which 

 the magnifying power can be increased without disturbing the 

 object under examination. 



Owing to their more perfect corrections, these lenses are speci- 

 ally adapted for photomicrography, which is becoming more 



