78 G. WILLIAMS ON THE USE OF POWELL'S ILLUMINATOR. 



the disc on the stage of the microscope, in case it should be desired 

 to examine the mount as an ordinary slide by illumination from the 

 sub-stage. The disc will require to be kept steady by a slight spring 

 ring to counteract the suction of the immersion objective. 



It will, I think, be seen that by mounting the diatoms on a small 

 disc instead of the ordinary slip, and placing it in the holder above 

 described, the whole of the frustules on the cover are brought under 

 the control of the observer, who, by mere rotation, can bring any one 

 of them into the necessary position, and thus ultimately succeed in 

 examining a large majority. 



Having selected a specimen, and rotated it into position, one of 

 the uprights of the disc-holder may chance to stand immediately 

 between the diatom and the light ; in that case, all that is necessary 

 is to turn the disc in its setting, and again rotate the holder until 

 the object is again in position ; a glance will show whether or not 

 the light will now pass between the uprights, if not, the turning of 

 the disc and rotation of the holder must be repeated until it does so. 



The turning of the disc and rotation of the disc-holder can be 

 easily managed with thumb and finger, under a quarter-inch objec- 

 tive ; indeed, with a well-made apparatus, but little difficulty has 

 been found even under a -^th, aided by a mechanical stage in good 

 working order. The rotation can, of course, be effected by the use 

 of the concentric motion of the microscope stage ; but, in practice, 

 it will be found far preferable to rotate the holder itself to approxi- 

 mately place the object, resorting to the concentric motion to effect 

 only the niceties of position ; the milled heads which work the 

 mechanical stage will then retain their normal situation (or very 

 nearly so) , at the right side of the microscope, and be, consequently, 

 much more convenient for the manipulation of the observer. 



