4 PRACTICAL MICROSCOPY. 



struments used in the New York University Laboratory of Biology 

 and Pathology. They were constructed by Schrauer, of this city, 

 costing about fifty-five dollars each. They are provided with a single 

 eye-piece, and Hartnack objectives Nos. 2 and 7, giving from. 30 to 

 400 diameters. Such an instrument is well adapted to the work of 

 normal and pathological histology, though a condenser * should be 

 attached below the stage and in the optical axis for high-power work 

 with immersion lenses, and especially for bacteriological research. 

 The stand is a rigid one, and if the height of the table upon which it 

 is placed, and the chair of the observer be in a proper relation, no 

 discomfort need be experienced in using the microscope in the verti- 

 cal position. 



ADJUSTMENT OF THE MICROSCOPE. 



The microscope should be placed in front of the observer, on a 

 table of such height that, when seated, he may, by slightly inclining 

 the head, and without bending the body, bring the eye easily over the 

 eye-piece. The slightest straining of the body or neck should be 

 avoided. The light should always be taken from the side, and 

 it matters little which side. Clouds or clear sky serve as the best 

 source of light for our present work. Always avoid direct sunlight. 

 If artificial illumination be employed though it is not advised for 

 prolonged investigation a small coal-oil flame may be tempered by 

 blue glass. 



ADJUSTMENT FOR ILLUMINATION". 



It will be observed that there are two mirrors in the circular frame 

 below the stage one plane and the other concave. The latter will 

 be employed almost exclusively in the work of this volume ; and its 

 curvature is such that parallel rays, impinging upon its surface, are 

 focussed about two inches from the mirror. It will also be noticed 



* A non-achromatic condenser, after the formula of Abbe, of Jena, is in 

 quite general use in this country. Its value has been very markedly increased 

 here by the addition of a rack-and-pinion motion. In use for high power work 

 with tissues, it is first placed so that the plane surface of the upper lens is in con- 

 tact with the under surface of the glass slip holding the object to be exam- 

 ined. Light is then reflected into the condenser as usual, excepting that the 

 plane mirror is employed. This will give a strong illumination, but too diffuse 

 for tissues. The light is then modified by diaphragms, or by racking the con- 

 denser downward until the best effect is secured. For bacterial search the 

 strong illumination is employed. This gives prominence to the stained mi- 

 crobes, as the other elements in the field are lost in the excess of diffuse 

 light. 



