COMPRESSOKIUM. 151 



there is a very large class, whose organization is so delicate as 

 to be confused or altogether destroyed by the slightest excess of 

 pressure ; and for the examination of such, an instrument in 

 which the degree of compression can be regulated with pre- 

 cision, is almost indispensable. Various plans of construction 

 have been proposed; but none among them appears to the 

 Author to present so many advantages as the one represented in 

 Fig. 50, the general plan of which was originally devised by 

 Schick of Berlin, but the details of which have been modified 

 by M. de Quatrefages, who has constantly employed this instru- 

 ment in his elaborate and most successful researches on the 

 organization of the Marine Worms. It consists of a plate of 

 brass between 3 and 4 



inches long, and from F'"' 50- 



1^ to 1^ inch broad, 

 having a central aper- 

 ture of from J to I of 

 an inch. This central 

 aperture is covered on 



its upper side by a Compressorium. 



disk of thin glass, 

 which may be cemented to the brass plate by Canada balsam ; 

 and the under side of it is bevelled away, so that the thickness 

 of the edge shall not interfere with the approach of the objective 

 to its margin, when that side is made the uppermost. Near one 

 extremity of the plate is a strong vertical pin, that gives support 

 to a horizontal bar which turns on it as on a swivel ; through 

 the end of this bar that projects beyond the plate, there passes a 

 screw with a milled head; and at the other end is jointed a 

 second bar, against one end of which the screw bears, whilst the 

 other carries a frame holding a second disk of thin glass. This 

 frame is a small circular plate of glass, having an aperture equal 

 in size to that of the large plate ; to its under side, which is flat, 

 a disk of thin glass is cemented by Canada balsam, while its 

 upper side is bevelled off as it approaches the opening, for the 

 purpose just now specified; and by being swung between pivots 

 in a semicircle of brass, which is itself pivqted to the movable 

 arm, it is made capable of a limited movement in any direction. 

 The upper disk with the apparatus which supports it, having 

 been completely turned aside round the swivel-joint, the object 

 to be compressed is laid upon the lower disk ; the upper disk is 

 then turned back so as to lie precisely over it, and by the action 

 of the milled-head screw, is gradually approximated to the 

 lower, to which the pivot movements of its frame allow it to 

 take up a parallel position, whatever may be the inclination of the 

 bar. As it is frequently of great importance to be able to look 

 at either side of the object under compression, the principal 

 plate is provided with two pins at the extremity farthest from 

 the milled head, which, being exactly equal in length to the 

 swivel-pin, aflford, with it, a support to the instrument, when it 



