290 E. M. KELSON ON METHODS OF ILLUMINATION. 



An illumination such as that by parallel, or nearly parallel rays 

 is to be avoided. Even that kind of illumination, now much in 

 vogue with photomicrographers, which may be termed "lantern 

 illumination," because the illuminating cone is focused upon the 

 front lens of the objective, is to be deprecated ; for it is only a 

 method of obtaining an evenly illuminated field at the expense of 

 loss of definition in the image. 



But how does a medical student or biologist examine an object? 

 He places his preparation on the stage and then fumbles about 

 with the mirror until he succeeds in obtaining an evenly lighted 

 field, and when he has got this he is quite satisfied. Let Ui^ 

 follow his example, put our work to the test, and then note the 

 result. Taking any object say a blow-fly's tongue, an inch 

 objective, and a low eyepiece let us fumble about with the 

 mirror until we have an evenly lighted field. If we now con- 

 centrate our attention upon one of the stronger hairs in the 

 object, and with the coarse adjustment put it within and without 

 the focus, it is ten chances to one that the out-of-focus coma 

 will rock from side to side, or up and down, in the field, which 

 shows us at once that our illuminating beam is not central. 

 Turning now from the strong hair to the delicate ones on the 

 membrane, we shall not find them imaged as sharp black spines. 

 as they should be. The student will naturally ask what he 

 should do. It would be unkind to refer him to one of the many 

 treatises on the microscope, for the only information he will 

 find on the subject will be : " Put it under, and, by moving the 

 mirror, obtain an evenly lighted field " ! The proper method of 

 procedure is very simple. Focus the object ; remove your eye 

 from the eye-lens and look at it, not through it, and, by moving 

 the mirror, bring the image of the light-source, be it window or 

 lamp-flame, central in the eye-spot, or Ramsden disc. Now,, when 

 the image is tested by focal alteration, the coma will spread out 

 equally on all sides of the image, and the delicate hairs will 

 appear like sharp little thorns. Naturally the image will be 

 inferior to that when a condenser is used, but a great difference 



