ELEMENTS OF MICROSCOPY. 125 



formed, and it is, therefore, clear that by moving the lamp nearer 

 to or farther from the mirror, we can so arrange matters that the 

 image of the lamp flame shall be formed upon the object which 

 we are examining. 



Farthermore, since we have to deal (i) with a cone of light 

 widening from the lamp and the mirror, and (2) with a cone of 

 light narrowing from the mirror to the stage, it is clear that if we 

 move the lamp farther from the mirror, and make the second cone 

 shorter, we make its angle wider, since the base, which is the size 

 of the mirror, always remains the same. Therefore, in working 

 with low powers, of low angle, we make the cone between the 

 mirror and the stage as long as we can, or as long as will do the 

 work satisfactorily, and with higher powers we move the mirror 

 closer to the object and put the lamp farther away from the mirror. 

 Unfortunately, in stands such as are likely to be in the hands of 

 many of those for whom we are specially writing, the distance of 

 the mirror from the stage is a fixture. The only method available 

 is then as follows : — Turning the mirror full on to the stage, 

 measure the distance between mirror and object. Then turn the 

 mirror round to face the lamp flame, setting the latter, with the 

 cardboard box by its side, at the distance found to exist between 

 mirror and stage. The lamp alone must then be moved away 

 until its image is sharply defined upon the bottom of the box, and 

 the distance between the mirror and the lamp flame thus found 

 by trial must be adhered to in working, in order that the rays 

 from the mirror may be brought to a focus upon the object. 



We fear that the details given above may appear to be some- 

 what uninteresting, but after all, the microscopist who does not 

 know a little of elementary optics is very much in the position of 

 a carpenter who does not know how to adjust his plane. He will 

 get shavings, probably, of some sort, but the resulting work will 

 be of no higher class than that of the microscopist who sets his 

 microscope up in a "happy-go-lucky"' fashion, without any regard 

 to the principles on which it is constructed, and of such, alas, 

 there are far too many. 



{To be continued.) 



