POWELL AND LEALAND's MICROSCOPE. 81 



by oblique ligbt, in order that, without disturbing tlie 

 illuminating apparatus, the eft'ect of the light and shadow 

 may be seen in every direction, whereby important addi- 

 tional information is often gained ; and, secondly, in the 

 examination of objects under polarized light — a class of 

 appearances being produced by the rotation of the object 

 between the prisms, which is not developed by the rota- 

 tion of either of the prisms themselves. The graduation 

 of the circular rack, moreover, enables it to be used as 

 a goniometer. In the improved form of this instrument 

 here represented, the whole stage-apparatus is made so 

 thin, and the opening beneath so large, as to permit the 

 employment of light of extreme obliquity ; and to enable 

 the mirror to afford this, it is mounted upon an extending 

 arm, the socket of which slides upon a cylindrical stem. 

 Below the stage, and in front of the stem that carries the 

 mirror, is a dovetail sliding-bar, which is moved up and 

 down by the milled-head shown at its side. This sliding- 

 bar carries what is termed by Mr. Eoss the " secondary 

 stage " (shown separately at b), which consists of a cylin- 

 drical tube for the reception of the achromatic condenser, 

 the polarizing prisms and other fittings as in Messrs. Powell 

 and Lealand's arrangement; it is here shown as fitted with 

 a condenser specially devised by Mr. T. Ross for the illu- 

 mination of a large field under low magnifying powers. 

 To this secondary stage, also, a rotatory motion is com- 

 municated by the turning of a milled-head ; and a tra- 

 versing movement of limited extent is likewise given to 

 it by means of two screws, one on the front and the other 

 on the left-hand side of the frame which carries it, in order 

 that its axis may be brought into perfect coincidence with 

 the axis of the " body." 



The mechanical movements and general finish of the 

 instrument is all that can be desired by the practised 

 observer, and tends towards that saving of time and 

 labour as essential in microscopical pursuits as in other 

 branches of science. Although our description is confined 

 to Mr. Ross's first-class microscope, other instruments 

 without the accessory arrangements described may be had 

 equally well made and suited to the means of the student, 

 ranging in prices from 121. 12s. upwards. 



G 



