90 



THE MICROSCOPE 



screwing the upper lens cell up and down into tlie tube they may 

 be sharply focussed (see page 67). 



An eyepiece with a movable pointer in the field of view, an 

 erecting eyepiece and a polarising eyepiece are described in 

 pages 72 and 73. 



Histology 



and 



pathology 



The Selection or Object Glasses and Eyepieces 



Advantage Where pricc is not an object, it is advisable to have a complete 

 of complete ggj^ Qf either achromatic or apochromatic object glasses, including 

 only one of the 2/3-inch apochromatic and one of the 1/6-inch 

 apochromatic. Each size has its special uses as previously described, 

 and they do not overlap. It is best to have apochromatic 

 object glasses and a complete set of compensating eyepieces. 

 At a time when many intermediate sizes of object glasses were 

 made, a selection was always necessary ; now that they have been 

 reduced to a smaller number of standard sizes, they are all of 

 great assistance to any observer. It is not, however, every 

 microscopist who can afford to buy a complete set, and in this 

 case the selection becomes a matter of importance. For work 

 on all subjects, all sizes will probably be eventually required, 

 though they need not be purchased at once. 



For histological and pathological work, the student is advised 

 by his teacher to purchase a 2/3-inch (16-mm.) and a 1/6-inch 

 (4-mm.) object glass and two eyepieces magnifying X6 and XlO, 

 adding a 1/12-inch oil-immersion for pathological work at a later 

 date. It is a question whether in some cases he might not do 

 better to start with a 1/3-inch (8-mm.) and an 1/8-inch (3-mm.) 

 oil-iramersion, adding a IJ-inch (32 -mm.) for very low- power 

 work if required. In such a case he should have three eyepieces, 

 X6, XlO, and Xl5, as the highest power eyepiece enables a 

 great deal of work to be done with the 1/3-inch (8-mm.) that would 

 generally be done with a 1/6-inch (4-mm.). The 1/8-inch (3-mm.) 

 oil-immersion is a very useful power. The 1/3-inch (8-mm.) is 

 only slightly affected by the thickness of the cover glass, and the 

 1/8-inch oil-immersion is not affected at all, so that the unskilled 

 observer is more likely to get the best out of his instrument with 

 these two powers. The addition at a later date of a 1/12-inch 

 apochromatic object glass and a high-power compensating eye- 

 piece makes a very perfect outfit. Where price is of great 

 importance, the cheapest outfit will be a 2/3-inch (16-mm.) and 

 1/6 -inch (4-mm.), as usually recommended. 



For biological work the same remarks apply to a great extent, 

 but the 2/3-inch (16-mm.) has the great advantage that, used with 

 an erecting eyepiece, it turns the instrument at once into a dis- 

 secting microscope. It is also sometimes troublesome to use an 

 oil-immersion with an unmounted specimen examined in water 

 under a cover glass, and a high- power dry lens is often preferred. 



Biology. 



