OP THE MICROSCOPE. 159 



stage ; over it is placed the slide. If the metal plate be 

 fastened to the stage by any contrivance, the object-slide 

 may be moved about at will without disturbing the prism. 



As we have already stated, the Wenham prism has a right 

 angle and two angles of 45 each. Prisms for the same purpose 

 have been made with unequal angles at the base, and with other 

 than a right angle at the apex, and this is sometimes advan- 

 tageous. 



It is, of course, obvious that the Wenham prism transmits 

 the rays with their direction unaltered. If we wish to condense 

 them, this must be done previously by means of mirrors or 

 lenses. Two illuminators, which combine the effects of the 

 condenser and the prism, have been devised by Mr. Wenham, 

 and we will now describe them. 



The Hemispherical Illuminator. Instead of a 



prism, Mr. Wenham has used a hemispherical lens, patched on 

 to the slide in the same manner as that described for the prism. 

 This lens concentrates the rays on the object, and as rays from 

 any direction will always enter the lens at right angles to some 

 part of its surface, more light and greater obliquity can be ob- 

 tained by it than by the prism. This device was described by 

 Mr. Wenham in the same paper in which he first described 

 the right-angled prism, but we believe it was first applied 

 by Mr. Tolles as an oblique illuminator for balsam-mounted 

 objects seen with immersion lenses. 



The "Half-Button." A still more efficient illuminator, 

 and one which is complete in itself, is shown in the engravings, 

 Fig. 48 being a side view, Fig. 49 a section, and Fig. 50 a per- 

 spective view of this little contrivance. It consists of a semi- 

 circular disc of glass of one-quarter of an inch radius ; the edge 

 is rounded and well polished to a transverse radius of one-tenth 

 of an inch, for the reason that the focus of a spherical surface 

 on crown glass falls within its substance to nearly three times 

 the radius, consequently the line of light Avill be in the most 

 concentrated position at one-twentieth of an inch above the 

 centre of the semi-disc, which distance is sufficient to reach ob- 

 jects mounted on slides of the usual thickness. The "half- 



