ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



119 



Usually in Microscopes the polariser is fitted to the substage ring 

 •which carries the Abbe condenser, and cannot be attached until the sub- 

 stage is lowered and the condenser is removed. For attaching the 

 analyser the Microscope-tube must be raised, the objective or nose-piece 

 removed, and the analyser screwed at the upper end to the tube, and at 

 the lower end to the objective or nose-piece. Both the substage and 

 the tube must be readjusted before the object can be viewed. The 

 change back again to plain illumination is equally laborious. These 

 operations not only consume several minutes each time the polariscope 

 is brought into service, but also, if often repeated, are ruinous to the 

 screw-thread and other parts of the apparatus. 



Fig. 21 shows Bausch and Lomb's Continental Microscope fitted 

 with this arrangement. The polariser (fig. 22) is carried on an arm 

 below the substage, and swings into position from the right, at the same 

 time forcing the iris diaphragm out of position to tbe left. The analyser 

 is mounted in the same manner as in petrographical instruments, the 

 prism being contained in a box which slides in the main tube, so that 



Fig. 22. 



when pushed to the right, the light passes through the prism, but when 

 pushed to the left, a round opening permits unobstructed vision. When 

 polarised light is desired, the polariser is pushed to the left and the 

 analyser box to the right, and the change to plain illumination is accom- 

 plished by the reverse operations. Either change can be made in less 

 ihan a second, without disturbing the adjustment either of the tube or of 

 the substage, and without damage to the instrument. Another advantage 

 is that the condenser may be used in conjunction with the polariser, 

 thus rendering the crosses on wheat, rye, barley, and some other feebly 

 active starches more distinct. Selenite plates are mounted in a metal 

 slip and are used on the stage. 



Watson & Sons' New Substage Condensers. — Fig. 23 represents 

 the dry parachromatic condenser lately introduced by Watson & Sons. 

 It has a total aperture of 1 • and an aplanatic aperture in excess of ■ 90. 

 Its power, 2/7 in., renders it suitable for critical and photographic work 



p. 93S) of a Microscope with analyser in box above the objective. In the volume for 

 J 888 (fia:. 40, p. 279) Messrs. Bausch anil Lomb's " Petrographical Microscope," with 

 this device, has already been described. Messrs. Swift also mount the analyser in a 

 slide above the objective. — Editor. 



