A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 201 



their means. If I should happily succeed in convincing one 

 such person of the utter fallacy of these notions, that practically 

 no study of optical principles of a dry-as-dust nature is requisite 

 in order to secure absolutely critical results, and that the 

 outfit necessary for the great majority of pui-poses is in- 

 expensive, and is probably already in the hands of most, I 

 shall feel that I have not trespassed on your patience or 

 claimed your indulgence in vain. 



For the attainment of a strictly critical image the only 

 essentials are: — 



(a) That the source of light (preferably the edge of a lamp 

 flame) be sharply and centrally focussed on to the object by 

 means of a sub-stage condenser of suitable aplanatic aperture, 

 the microscope being either pointed directly to the illurainant, 

 or, if the angle at which the instrument must be inclined to 

 etfect this should from any cause prove inconvenient, the plain 

 mirror may be requisitioned to reflect the light into the 

 condenser. Any light source which does not lend itself to being 

 sharply focussed on the object, such as the incandescent electric 

 lamp, should be avoided. 



(b) That the aperture of the substage condenser diaphragm 

 be at least large enough to illuminate three-quarters of the 

 diameter of the back lens of the objective employed. 



(c) That the microscope tube length be such a-s to exactly 

 compensate for variations in the thickness of the cover glass, 

 this correction being readily effected by pulling out or sliding 

 in the draw-tube, and noting the varying clearness of the image. 

 The best point is easily determinable when a large illuminating 

 cone is used. 



With due attention to these three requisites good results are 

 invariably attainable. 



As to the instrument and indispensable accessories, it need 

 only be remarked that our opticians now construct admirable 

 small microscope stands, provided with good flne adjustments, 

 at a cost of from <£4 to .£6 ; that the semi-apochromatic Leitz 

 objectives are both cheap and most satisfactory in their per- 

 formance when used in conjunction with very large illuminating 

 cones, and that an aplanatic substage condenser is procurable 

 at a trifling outlay. With such an outfit, costing in all not 

 more than XIO, results of an exceedingly high order, with low 



