334 



ACCESSORY APPARATUS 



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

 jective and the Wenhain 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 

 list a nee for the focus of objectives of very low power to be shortened 

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



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



Fici. '279. 



FIG. 280. 



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

 and the Beck stands. 



A modification of t/te parabolic reflector was devised by Dr. Sorbij, 

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

 the microscopic structure of metals. It consists of a parabolic 

 reflector, iii 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 readilv used, 

 as the plane reflector is attached to an 

 arm so that it can be swuiiif out of the 



l-'li.. '2S1. Sorliy's inoilificjitifiii of 

 lllc p.llMllolic ivtlrrtor. 



way when not required, as shown in the figure. 



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

 sections of steel not otherwise attainable. 



No opaque illumination, however, has yel surpassed the venerable 



Lieberkiihn ; the besl experts freely admit thai the finest critical 



1(1 be obtained by this met hod of illumination are secured 



'be LieberUiihn. This mode of illuminating opaque objects is 



b\ means of a >m.-dl concave speculum reflecting directlv down upon 



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



