TESTING THE MICROSCOPE ITS USES. 9 



It is a matter of some indifference which color prevails. 

 These defects are best seen by observing a bubble of air in a 

 fluid specimen. The prevailing color is seen at the periphery 

 of the bubble. 



Thirdly, all objects in the field should appear with equal 

 distinctness, whether at the periphery or in the centre. If a 

 fine powder, such as lycopodium be strewn over the field, the 

 granules should be seen as distinctly at the edges as at the 

 centre ; an ordinary thin section of any microscopic object will 

 also exhibit this defect, if it exist. 



Fourthly, the glasses should have good penetration. This 

 enables the observer to see the general aspect of bodies better, 

 though it may not make him see objects quite as sharply ; the 

 former depending upon a large angle of aperture, and the latter 

 (definition) upon a small one. 



To be able to have at the same time both great resolving 

 and great defining power is the highest desideratum, and it has 

 been the merit of our American makers to increase the angle 

 of aperture and still maintain a high defining power. 



For ordinary histological purposes, a lens that will show 

 the oscillatory movement in the mucous or salivary corpuscles 

 is sufficiently high for practical purposes. This is accomplished 

 by the ordinary student's one-fifth of Grunow, for example. 

 If, however, we are studying the delicate intercellular sub- 

 stance of the brain and connective-tissue corpuscles, bacteria, 

 etc., a somewhat higher power is needed. 



For such studies it is desirable to have an immersion lens, such as the No. 10 

 or 12 Hartnack or Prazmowski, or a ^ or ^ of other good makers, such as 

 Wale, Tolles, etc. 



In using these high powers it is necessary to place a single drop of water 

 on the anterior lens and depress the tube imtil the drop touches the circle 

 or cover. The drop of water utilizes light that would otherwise be lost, mag- 

 nifies slightly, and corrects, so that the image is made brighter and more 

 distinct. 



The new oil immersion of Zeiss is highly recommended by Woodward of 

 Washington. In using such a lens, a drop of oil is substituted for water. We 

 are hardly yet prepared to decide whether oil is preferable on the whole to 

 water. 



How to illuminate the microscope. In doing ordinary 

 microscopic work it is best to use day -light, such as is reflected 

 from a clear sky. It is not well to use direct sun-light, but to 



