50 PRACTICAL MICROSCOPY. 



request, and much work has been done with them. This 

 has induced several English makers to offer a series of low- 

 angle objectives at a cheap rate, in addition to the really 

 good glasses which the same opticians turn out. Browning, 

 Wray, Watson, Swift, Dancer, Collins, and Parkes make 

 very excellent and cheap lenses for histological work. 



It sometimes happens that very good powers may be 

 met with second-hand, or from the workshops of neglected 

 makers ; but the selection of a good glass from several of 

 average quality, though it can be done by an optician, or a 

 microscopist who has mastered the elements of science, is 

 not a very easy task, and one which certainly cannot be 

 performed by a novice on his first purchase of an instru- 

 ment. 



Be very careful in purchasing objectives. Don't buy 

 trash, or any objective simply because it is cheap. Wait 

 until you can meet with a really good glass at the price 

 you are disposed to give, or be satisfied with the powers 

 you already possess. 



For botanical and general work, especially if the student's 

 means are limited, the inch and J-inch objectives will 

 be found sufficient ; the 2-inch, and afterwards the J-inch, 

 may be added, if required ; following, if necessary, with 

 the ^-inch and ^-inch. A 4-inch power is used by some 

 for the examination of wood sections, whole insects, etc. 

 A cabinet will generally contain the following objectives by 

 the time the owner considers it furnished : 4, 2, i, J, , 

 ! -TO which yield degrees of amplification varying from 

 12 to 800 diameters with the A eye-piece, and 24 to 1600 

 with the C. 



The low powers are constructed in several ways, ac- 

 cording to the aperture desired. Those of greater ampli- 

 fication than the i-inch are made either as triplets or of 

 two pairs of lenses placed at certain distances apart, as all 



