ZOOLOGY AND BOTANY. MICROSCOPY, El'C. 353 



If the substance to be examined is a powder or a body which can only 

 be placed horizontally, the inclined mirror and small stage (fig. 68) are 

 used. Although silver is transparent to ultra-violet in the neighbour- 

 hood of o20 fj-fx, yet it is found that this method is usually successf til. 



The author has found the apparatus answer admirably for demon- 

 strating the properties of fluorescence and phosphorescence to a large 

 audience. He has obtained excellent results from such objects as 

 fluorescent varieties of Jena glass, cathodic rays and gems. 



Methods of Illumination.* — By the term "mirror illumination," 

 says E. M. Xelson, is meant the illumination of an object by transmitted 

 light with a plane or concave mirror without any substage condenser. 

 This form of illumination was, up to a few years ago, very extensively 

 employed, the use of a condenser, especially upon the Continent, being 

 quite exceptional. The plane mirror is for use in conjunction with a 

 substage condenser, and the concave for use by itself. 



The ideal illumination for transmitted light is obtained when the 

 object is at the apices of two conjugate solid cones of light. An illumi- 

 nation such as that by parallel, or nearly parallel, rays is to be avoided. 

 Even that kind of illumination, now much in vogue with photomicro- 

 graphers, which may Ite 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 illumi- 

 nated field at the expense of loss of definition in the image. 



But how does many a student 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. As many treatises on the Microscope say, 

 " Ptit it under, and, by moving the mirror, obtain an evenly lighted 

 field " ! But 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 

 disk Now, when the image is tested by focal alteration, the coma 

 will spread out equally on all sides of the image, and 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 will be noticed 

 between those obtained with a centred and decentred mirror-image in 

 the Ramsden disk. The student should remember that it is far better 

 to have a centred illumination, even at the expense of an incompletely 

 lighted field, than an evenly illuminated field and decentred illumination. 



To illustrate this subject further, let the flat of the flame of a paraffin 

 lamp be used as an illumiuant, then, with concave-mirror illumination 

 improperly arranged by a f umbler. the image in the Ramsden disk would 

 very probably appear as in fig. 71, No. 1. The image of the flame 

 resembles a decentred slit of light, notwithstandhig that the flat of the 

 flame is presented to the mirror. 



It is this decentring of the illumination which causes the coma to 

 rock upon focal adjustment. It is the asymmetrical arrangement of the 



* Journ. Quekett Micr. Club, ser. 2, xi. (1911) pp. 289-98 (1 fig.). 



