664 SUMMAEY OF CUREENT RESEARCHES RELATING TO 



and also the vertical divisions upon a small scale attached to or 

 engraved upon the lower stage-plate H. This vertical scale can of 

 course be placed in any other position either at the side or top of the 

 stage ; but I think the position I have indicated is the most con- 

 venient one. 



In the case of a mechanical stage, the vertical screw movement 

 could be used, the horizontal one being brought to a fixed position by 

 means of a line marked upon it. 



For use with a Microscope with a stage moving in all directions, 

 my friend Mr. J. D. Hardy, of the Quekett Microscopical Club, has a 

 small but ingenious addition to make this finder useable. 

 'In one of the smell circular openings in which the stage- 

 plate moves, he attaches a piece of metal G, Fig. 148, by 

 means of a screw. This serves at once as a vertical scale 

 and also makes the stage-plate move vertically in a fixed 

 line, without which the finder could not be used. 



The use of the finder is so obvious, I think, as to need 

 scarcely any description. The object (say a slide of diatoms) 

 is placed on the stage and moved about until any particular form is 

 in the centre of the field ; and the figures (say 11-7) are at once 

 apparent and ready for record. By reading the intermediate spaces 

 as 11 "5-7 "5, or nearer, great accuracy may be obtained; divisions 

 of -^-^ of an inch on the arc will represent y^^ or the -^^-^ on the 

 slide according to the distance from the centre E. Objects under 

 powers of 1000 or 2000 diameters may be centered with the utmost 

 accuracy, and the exact stiflhess in motion can at all times be given 

 by the adjustment of the screw centre E. 



In one of the German forms a racked arc worked by a milled head 

 is used, when of course very great accuracy may be obtained ; but for 

 ordinary purposes I think this simple plate will be found to answer 

 most required piu'poses, and it could be easily fitted to almost any 

 form of Microscope. I do not make any comparison of this method of 

 object-finding with any of the other ingenious appliances in use, but I 

 think it will be found useful and perhaps applicable in some cases 

 where other methods might not be so convenient. Except in the case 

 of putting on a large trough, there is no need even to remove it, as it 

 makes an agreeable working stage." 



Wenham's Disk Illuminator. — Fig. 149 (natural size) shows a plan 

 of mounting this disk,* devised by Mr. Wenham specially for Boss's 

 improved Microscope. The disk is held between two small vertical 

 plates attached to the cap of a cylinder that rotates by screwing into a 

 metal ring fitting beneath the object-stage ; it is thus entirely free of 

 the substage and mirror, and when adjusted, forms part of the object- 

 stage. The power of rotation (a most important element to develope 

 the best effects with the device) is provided by the screw fitting, 

 which also serves to adjust the disk at the required level for immer- 

 sion contact with the base of the slide. The rotating-plate is suitably 

 cut to allow a large angle of obliquity to the incident rays. 



* See this Journal, iii. (1880) pp. 145-7. 



