COMPRESSORS 



347 



efficient form ; we illustrate it in fig. 300, but on a reduced scale. 

 The bevelled glass in this also is kept small, with respect to the 

 size of the cover-glass, and it acts with perfectly parallel pressure 

 between the two glasses, which in delicate work is of considerable 

 importance. 



The cover-glass is held on an arm which screws down on a vertical 

 post against a spring ; as the screw is raised the spring raises the 

 cover-glass, and by an ingenious spring catch it is kept central with 

 the glass-plate floor. This can nevertheless be released, and the entire 

 cover can be turned aside to put on a 

 fresh object, clean, and so forth. It is 

 simple, light, and, being parallej, can 

 be used with the highest powers. * 



Messrs. Beck and Co. have for 

 many years made an admirable parallel 

 compressor, but its weight and cost 

 were somewhat prohibitive of its use 

 generally ; the firm have now overcome both difficulties by the intro- 

 duction of a new form which is most useful and fully accomplishes 

 its work. 



This compressor was designed by Mr. H. R. Davis, and is 

 specially intended for the examination of living objects. It consists, 

 as shown in fig. 301, of a lower ebonite plate A, which has a circular 

 hole in the centre, and which is recessed to receive a circular brass 

 ring B. This ring rests loosely in the recess. On the recessed 

 portion of this plate A is carried an oblong thin glass which is 

 held in position by two screws, one of which appears at C. Two 

 end plates D D slide on to the plate A, and hold the ring B loosely 

 in position, allowing it to be revolved by means of its milled flange, 

 which projects at E. Within the ring B is screwed a brass disc F 

 which carries the upper thin glass which is attached by the screws 



FIG. 300. Rousselet's compressor. 



G G 



FIG. 801. Beck's new compressor. 



G G. The screws G G and C, fitting into holes in the lower plate 

 A and the disc F respectively, prevent the disc from revolving, and 

 when the ring E is turned, the two thin glasses are moved towards 

 or away from one another. 



The slides D D and the ring B, together with the disc F, are 

 removed for arranging the object on the lower cover-glass, and 



