444 



Mr. W. Crookes on a New Arrangement of [May 27, 



The two additions to the microscope consist of the substage with slit 

 &c, and the prisms in their box. The substage is of the ordinary con- 

 struction, with screw adjustment for centring, and rackwork for bringing 

 it nearer to or withdrawing it from the stage. Its general appearance is 

 shown in fig. 1, which represents it in position. A B is a plate of brass, 



Fig. 1. 



sliding in grooves attached to the lower part of the substage ; it carries 

 an adjustable slit, C, a circular aperture, D, 06 inch in diameter, and an 

 aperture, O, J inch square, A spring top enables either the slit or one of the 

 apertures to be brought into the centre of the field without moving the eye 

 from the eyepiece. Screw adjustments enable the slit to be widened or 

 narrowed at will, and also varied in length. At the upper part of the sub- 

 stage is a screw of the standard size, into which an object-glass of high 

 power is fitted. E represents one in position. I generally prefer a ^-inch 

 power ; but it may sometimes be found advisable to use other powers here. 



