334 



ACCESSORY APPARATUS 



a commendable plan, for it increases the distance between the ob- 

 jective and the Wenham binocular prism ; and as the binocular is 

 specially suited for the kind of object usually examined with this 

 speculum, this increased distance, acting detrimentally on the be- 

 haviour of the binocular prisms, and causing the available racking 

 distance for the focus of objectives of very low power to be shortened 

 by the width of such collar, is to be avoided. 



The best plan without doubt is to attach the speculum to a fixed 



FIG. 279. 



FIG. 280. 



part of the stand, as is done in the Powell and Lealand, the Ross, 

 and the Beck stands. 



A modification oj the parabolic reflector was devised by Dr. Sorby, 

 and has proved to be very useful in certain investigations, such as 

 the microscopic structure of metals. It consists of a parabolic 

 reflector, in the centre of which, in a semi-cylindrical tube open in 

 front, is placed a small plane reflector which covers half of the 

 objective, and throws the light directly down upon the object and 



back through the other half. It is 

 shown in fig. 281 with the cylinder in 

 place, and in the dotted lines with the 

 same turned out. This arrangement 

 ^ allows of two kinds of illumination, 



oblique and direct, being readily used, 



Fl - ^SliV^etctf ' 1 f the P lane '^tor is attached to an 



arm so that it can be swung out of the 

 way when not required, as shown in the figure. 



Dr. Sorby was able to get results in the examination of polished 

 sections of steel not otherwise attainable. 



No opaque illumination, however, has yet surpassed the venerable 

 Lieberkiihn ; the best experts freely admit that the finest critical 

 images to be obtained by this method of illumination are secured 

 by the Lieberkiihn. This mode of illuminating opaque objects is 

 by means of a small concave speculum reflecting directly down upon 

 them to a focus the light reflected up to it from the mirror ; it was 



