INTRODUCTION 3 



Since, with an uncorroctod and unadjusted condiMisci-, 

 glare prevents the attainment of a '^lo aperture, an impor- 

 tant step in microscopy is the adjusting of the aspheric 

 or achromatic condenser for the distance of the lamp, and 

 the thickness of the slide, combined with the constant use 

 of the condenser with water immersion for high powers. 

 This includes the learning of the ring test. 



A fifth step in microscopical training is the correct 

 use of light screens and light filters. When the iris of the 

 condenser is put at the right aperture, the light may be too 

 intense. In this case, a neutral screen or a colored light 

 filter must be interposed. A set of yellow-green light 

 filters, for reasons to be subsequently given, provides the 

 best screen in many cases. Without some such regulation 

 of the intensity of the light for different objectives, a 

 %o condenser cone cannot usually be obtained. Without 

 yellow-green screens, one of the great aids to microscopy 

 is absent. 



For high powers especially, it is demonstrable that 

 there is a light haze or glare over the object, when the 

 aperture of the condenser approaches that of the objective, 

 and every other adjustment is correct. This glare clears 

 away on sufficiently contracting the image of the source 

 of light on the slide by a circular diaphragm close to the 

 source. A 3-millimeter diaphragm is usually suitable for 

 the highest useful magnifications, with a lamp distance of 

 about 25 centimeters. Training in the correct use of such a 

 diaphragm is apparently essential for the production of 

 the best microscopical images. 



Since a water-immersion objective can give better 

 images of objects at depths of more than a few microns 

 below water than an oil-immersion objective of similar 

 aperture, the microscopist who is concerned with biology 

 will, with advantage, train himself in the use of the correc- 

 tion collar of a water-immersion objective, in order to get 

 the best images in watery media. 



Objects near or below j 5 micron across, w^hich cannot 

 readily be differentially stained, are often well seen in a 



