THE MIRROR 



191 



a ^th of N.A. 1'4 or 1'5, would be obtained by using a precisely 

 similar objective as a condenser, with its back lens stopped down by 

 a slotted stop, the slot being of the size of the peripheral sector re- 

 quired to be illuminated. The cone of illumination would precisely 

 equal that taken up by the objective, and would be of maximum 

 intensity. 



Xow these conditions are more nearly approached by a high -class 

 achromatic condenser of great aperture and of homogeneous construc- 

 tion than by any other means. 



The value of oblique illumination is not here in question ; what 

 we believe clearly shown is that, however much may have been done 

 by oblique illuminators dependent on swinging sub-stages, and the 

 like, the same things can be better done with immersion condensers of 

 great apertures and perfect corrections. 



The swinging sub-stage, with these considerations as well as all 

 other * oblique illuminators ' of its order is a useless and defective, 

 not to say deceptive, adjunct to the microscope ; and this judgment 

 has so far obtained amongst practical microscopists as to cause the 

 virtual disappearance of the swinging sub-stage. It has no valid 

 function is unfruitful specialisation in fact which does not pro- 

 mote the progress of either the instrument or the worker. 



And this will apply to those complex forms of microscope known 

 as * radial,' ' concentric/ and those provided with stages that revolve 

 or ' turn over ' in an axis at right angles to the optical axis of the 

 microscope. 



In addition to the features enumerated hitherto, a complete sub- 

 stage should also be provided with a rack-and-pinion rotary motion ; 

 that is only really needed in order to use the polar iscope. For the 

 purposes of its successful employment this is important, but other- 

 wise its use is very limited. 



VI. The mirror is also an indispensable part of a complete 

 microscope. In a first-class stand it should be plane and concave 

 and from 2^ to 3 inches in diameter. It may be mounted on either 

 a single or a double crank arm. In any microscope, if there be 

 only one mirror, it should be concave. This mirror, from its curve, 

 has a focus, a point in which the reflected rays all meet ; and the 

 mirror should not be fixed, but so mounted that it may be focussed 

 on the object. 



The plane mirror is sometimes found to give several reflexions of 

 a lamp flame at one time ; we find a very efficient explanation of 

 them in a paper by Mr. W. B. Stokes in Vol. VI. of the second series of 

 the Journal of the Quekett Micro. Club, p. 322 (1896). His idea of their 

 origin is explained in fig. 154. A is the glass surface, B the silver 

 surface, O the object, and E the eye. In the direction 1, 2, 3 appear 

 the first three images. No. 1 is from the glass surface, No. 2 from 

 the silver, and No. 3 is from the silver and air surfaces. 



Move a card along A towards 1 , and No. 3 disappears first, No. 2 

 immediately after, and No. 1 when the card reaches that point. 

 This being their origin it may be asked how the images can alter 

 their position when the mirror is revolved in the plane of A. They 

 cannot ; the mirror A B has parallel surfaces, but microscope mirrors 



