432 The Eye and the Microscope. 



of the microscope as will not injure the eyes, may be varied 

 at the pleasure of the student, but it is well to begin with those 

 that can be shown with a low power, and which, either singly 

 or in groups, occupy all the field. Another class of objects do not 

 want more magnification for their efficient display, but are not 

 big enough to occupy so large a space. The tarsal joints of the 

 feet of many insects, their spiracles and mandibles, belong to 

 this category. Now, if such objects are surrounded by a flood of 

 bright light, they will not be distinctly shown, and the eye of 

 the observer will be uncomfortably affected. In these cases 

 the object should be arranged exactly in the centre of the 

 field, as a symmetrical appearance is more agreeable and less 

 fatiguing than the aspect of anything that looks askew. In 

 the next place, the light must be moderated by obliquity, 

 by the spermaceti paper, or by some other mode. The least 

 troublesome of these methods is the spermaceti paper, or a 

 piece of semi-transparent white glass made for the purpose. 

 If the object needs a very strong light to make it sufficiently 

 transparent, the best plan is to use an achromatic condenser 

 and allow a sharp pencil of light to reach it through one of the 

 smaller stops. The value of the achromatic condenser for 

 showing difficult objects is well known, but its advantage in 

 saving the eyes is less recognised than it deserves. Many 

 things, such as portions of insects, can be seen distinctly with- 

 out it, though with a fatigue which is avoided by its use. 

 When employed with low powers the usual plan is to remove 

 the upper part of the combination so as to bring its focal 

 length below that of the objective employed. In many cases, 

 however, it is better to use the whole, which we will suppose 

 to be a quarter-inch power, and to throw it out of focus, so that 

 the rays will cross before reaching the object. This plan, with 

 the use of one of the smaller holes or stops, gives enough light 

 for many purposes and prevents over excitement of the eye. 



No student who wishes to work efficiently with a quarter- 

 inch, a fifth, or a higher power, should begin with either of 

 them; but first learn to use an inch or two-thirds with the 

 various methods of illumination which we have indicated. If 

 premature efforts are made to obtain considerable magnification, 

 disappointment will inevitably result, and so far as what Dr. 

 .Kitchener termed the " economy of the eyes" is concerned, it 

 should be remembered that every increase of power augments. 

 the difficulties of obtaining a clear and pleasant view. An 

 experienced manipulator will show a properly selected and pre- 

 pared object with the highest objectives without any sacrifice of 

 convenience or distinctness; but it is only after considerable 

 practice that a beginner will be really successful with a quarter- 

 inch. The popular notion of the necessity for magnification is 



