LABORATORY WORK 



NOTE. The experiments described below will be found to vary greatly in their 

 difficulty and in the cost of the necessary apparatus. But I have not thought 

 it desirable to omit fundamental ones on either of these grounds, since their 

 performance is possible in many existing laboratories, and should become so in 

 the future to a much wider extent elsewhere. 



The student may require the actual co-operation of the teacher in some 

 instances. Certain of these may become demonstrations. It is desirable, 

 however, that the student should, if possible, repeat the experiments for himself. 

 This would naturally depend on the time available. 



If the course is held in a completely equipped laboratory, the teacher may 

 well be able to supplement the exercises given here. 



CHAPTER I 



The Microscope. Since this instrument will be very frequently 

 in use, it is well to understand, in their main outlines, the principles 

 on which it is constructed. A few experiments with lenses will 

 also give information needed for the study of the eye at a later 

 time. 



Take a biconvex lens of short focus, say about an inch, and note 

 that a large image of an object placed close to it can be projected 

 on to the wall of the room. The object and the image are at 

 conjugate foci, and there will be found to be numerous pairs of 

 such positions. The size of either is in proportion to its distance 

 from the lens. 



An appropriate object is a small black cross on a piece of 

 ground glass. The glass is illuminated by a bright light behind it. 



For many purposes, a small hand-feed arc-lamp is almost a 

 necessity. The best type is that in which the carbons are at right 

 angles to each other, since the bright crater in the positive carbon 

 is not obscured by the negative. The addition of a condensing 

 lens enables parallel rays to be obtained, and also the illumina- 

 tion of a small area with intensity. 



The lens of short focus used in the experiment above represents 

 the objective of the microscope, at that end of the tube nearest the 

 object examined. But the great distance of the large real image 

 makes this an impossible method of obtaining the magnification 

 desired. If, however, we place a second convex lens in the course 



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