30 Modern Microscopy 



sub-stage condenser for critical work. Opticians favour 

 a rectangular rather than cylindrical tailpiece to carry the 

 mirror gymbal ; the reason for this is a little doubtful, but 

 there is probably no distinct advantage in one over the 

 other. Where, however, a cylindrical tailpiece is provided, 

 it will be obvious that the mirror could quickly be swung 

 round out of the axis of the microscope and so obviate the 

 necessity for the swinging of the tailpiece itself, but this is 

 quite a minor consideration. 



The Mirrors should be plane and concave, hung in a 

 gymbal, giving universal movements, and have a means of 

 adjustment to focus in a vertical direction. The plane 

 mirror is always used with the condenser, spot lens, etc., 

 and with very low-power objectives, but the concave, when 

 the condenser is not employed and the maximum amount 

 of light is desired. 



A constant source of trouble and annoyance is an imper- 

 fectly worked plane mirror, which will give several reflec- 

 tions of the image of the lamp flame. The plane mirrors 

 usually fitted invariably do this. It should be insisted, in 

 a microscope with which high-class work is to be done, 

 that a parallel-worked mirror should be supplied. It is a 

 trifle more costly than the ordinary kind, but the additional 

 ■efficiency, on account of the reduced number of reflections, 

 is very appreciable. The parallelism of a mirror may be 

 tested by holding it just below the level of the eye in the 

 direction of a row of objects, such, for instance, as chimney- 

 pots ; and on observing the reflections, each subject should 

 stand out singly and clearly. If the mirror is not parallel- 

 worked several reflections of the same object will appear 

 superimposed in the mirror. 



Care is needful in the use of the concave mirror, if the 

 best result is to be obtained with it. It should be so 

 arranged that the apex of the cone of rays that it transmits 

 may be exactly in focus on the object. Many microscopes 

 are provided with mirrors that are unsuited to the instru- 

 ment, being either too long or too short in focus, and con- 



