170 HOW-TO SEE WITH THE MICROSCOPE. 



which resolved Amphipleura pellucida under these try- 

 ing circumstances, when used in the ordinary way with 

 this or other test-objects, displayed an exquisite perfec- 

 tion of definition which it would be hopeless to expect 

 to attain with objectives of less angular aperture. 



"As it is no part of my purpose in this communication 

 to provoke ill-tempered discussion of the merits of indi- 

 vidual makers, I will not append a list of the results 

 obtained with the various immersion objectives I have 

 tried in this way. The apparatus can be constructed 

 for a few shillings, and those who take the trouble to 

 use it will soon see to which of the three classes any 

 particular objective they may test belongs." 



Subsequent to the date of the reading of the preceed- 

 ing communication, Dr. Woodward proposed some slight 

 changes in the form of his prism. Having had consid- 

 erable experience with the prism as now used, we append 

 the following description, and also the manner of work- 

 ing the instrument. 



Essentially, it consists of a triangular prism of crown 

 glass. In the form adopted by Dr. Woodward the ob- 

 tuse angle is 98 and the two acute angles 41 each. 



The prism may be used unmounted, by simply attach- 

 ing the same to the under surface of the slide containing 

 the objects to be examined, a drop of glycerine or oil 

 of cloves serving to secure optical contact, and at the 

 same time acting as cement to retain the prism in place. 

 Notwithstanding this is the arrangement employed by 

 many observers, it is but a faulty plan, in fact, a regular 



