154 PKACTICAL PHOTO-MICKOGKAPHY 



In using illuminants of high power, under no circumstances 

 should the attempt be made to observe directly the image in 

 the microscope, unless some dark glass or similar light modifier 

 is placed in any convenient position between the illuminant 

 and the eye. The advantage of the method of carrying out the 

 various adjustments by means of the projected image on an 

 opaque screen becomes further evident in this case, as there is 

 then no possible risk of any harm to the eyes. This is a point 

 that requires careful consideration when working with illumi- 

 nants of the type of the electric arc, or the quartz mercury- 

 vapour lamp. From both of these a very considerable emission 

 of ultra-violet light takes place which, even with an exposure 

 of a minute or two, will cause very painful and even serious 

 inflammation to the eyes. They should in all cases be efficiently 

 screened off so that no direct light reaches the observer. 



Correction for Thickness of Cover-glass and Adjustment 

 of Tube-length. A point of great importance in using high 

 powers particularly those other than immersion objectives 

 and one which affects the quality of the image to a very marked 

 degree, is the thickness of the cover-glass over the object. 

 A flat refracting surface behaves to some extent like a spherical 

 one ; in fact, the flat surface may in theory be regarded as a 

 spherical surface of infinite radius of curvature. It therefore 

 suffers, although to a less extent, from the same defect as such a 

 curved surface ; oblique rays are refracted to a greater extent 

 than those situated more centrally. If an object is mounted 

 in Canada balsam, it results that light emitted from any one 

 point in the object, when it emerges from the surface of the 

 cover-glass into air, would proceed not as if it were coming from 

 one point, but from a succession of superimposed points, and 

 the effect would therefore be that the rays of greater obliquity 

 appear to emerge from a different point from those that were 

 less oblique. Fig. 55 diagrammatically illustrates this point. 

 Let the horizontal line FG represent the top surface of a cover- 

 glass, the object being mounted in Canada balsam beneath it. 

 If light were being emitted from a point in the object A, it 

 Would appear not as if it were from A only, but also as if it were 

 emitted from the points B, C, or D, according to its obliquity. 

 This example is, of course, hypothetical and exaggerates the 

 actual conditions for purposes of demonstration. The practical 



