INTRODUCTION. XX111 



2. Students' Microscope, II. A, is figured on p. 8 and described in the 

 text. With the appliances there mentioned, and including objectives 

 3 and 7 and eye-pieces 0, II. and IV., giving a range of magnification 

 from 46 to 625, price 9 15s. 1 



Most, or all, of the above makers, English and foreign, 

 manufacture microscope stands of cheaper quality than the 

 above ; it must, however, be borne in mind that accurate 

 observation needs an instrument which is capable of it, and 

 while there is, and ought to be, every desire to keep the cost 

 within reasonable limits, true economy does not consist in 

 purchasing an instrument which may be a constant source 

 of dissatisfaction, and may have to be discarded directly the 

 student emerges from swaddling clothes. Preferably, there- 

 fore, the stand which is purchased should be adapted to the 

 receipt of optical apparatus other than the mere eye-piece 

 and objective ; should have a jointed back, and be thoroughly 

 steady in any position ; the adjustment should be easy and 

 true, and if the body or draw-tube is twisted, any object 

 observed should not be thrown out of centre ; the mirror 

 should be plane and concave, and should have a long, jointed 

 arm ; and the stage should be constructed for the reception 

 of a 'substage condenser, and should be provided with an 

 iris diaphragm. Still more essential is it that the special 

 optical parts, the eye-pieces and objectives, should be good. 

 They should let through the largest possible amount of light 

 (the diaphragm will easily control its quantity, if needed), 

 and there should be absence of colour, both round the ex- 

 terior of the field of view, and round any object, .or particles 

 of dust, in focus. The field should be flat, so that a small 

 object moved from one part to another alters neither in dis- 

 tinctness, form, nor size. The high power objectives should 

 have a fair working distance from the object, or the thickness 

 of the cover-glass, to be hereafter noted, may become a 

 matter of great importance. Lastly, a double nose-piece of 

 accurate construction should be provided, and the objectives 

 should have their settings so graded that in changing from 



1 1 (Ed.) have a number of these instruments in use in my own laboratory 

 and think very highly of them. 



