ACCESSORY APPARATUS. 105 



tion of such obliquity to the optic axis of the Microscope, that even its 

 axial ray shall fall upon the object-slide at a very low inclination as in 

 the Ross-Zentmayer Microscopes ( 59, 72), and* in the arrangements of 

 Messrs. Beck ( 75) and Mr. Swift ( 68). It is considered by Mr. Wen- 

 ham, that there is no better method of utilizing this arrangement, than 

 by making the Sub-stage carry an ordinary Objective of about 1-inch 

 focus, and throwing its pencil upon a hemispherical lens of half an 

 inch diameter, the plane side of which has a film of glycerine interposed 

 between itself and the object-slide. The lens may either be held in this 

 position by its own adhesion, or it may be so fitted into a thin stage, 

 that its plain surface shall lie flush with the surface of the object- plat- 

 form. This (as also the Disk-Illuminator to be next described) may be 

 made to work well with any form of Students' Microscope, which, like 

 Wale's ( 60), has a thin stage and a mirror so swung as to be capable of 

 reflecting rays of great obliquity. For the illumination of objects by a 

 line of light thrown upon them very obliquely, Mr. Wenham has devised 

 the simple Illuminator shown in Fig. 76. This consists of a semi-circu- 

 lar dish of glass (somewhat resembling the half of 

 a button) of half an inch in diameter, the sides of 

 which are flattened, while the circular edge is 

 rounded and well polished to a transverse radius 

 of l-10th of an inch. This concentrates the light 

 thrown upon any part of its circumference, upon 

 an object mounted on a slide of the usual thick- 

 ness, with whose under side it is brought into Wenham's Disk-niummator. 

 immersion-contact by the intervention of either 



water, glycerine, or a more refractive oil. As it should be so fitted 

 to the Microscope as to illuminate the objects from any azimuth, it should 

 have its flat sides grasped in a clip, which may either be mounted on the 

 Sub-stage, or attached to under side of the Stage in either case having 

 its diametric section brought up to the under surface of the object-slide. 

 By giving rotation to the object, the illuminator remaining fixed, the- 

 illuminating beam may be made to cross the former in any direction that 

 is fitted to bring out its markings. With this simple Illuminator, even 

 Amplnpleura pellucida may be resolved without the aid of a Condenser, 

 the mirror alone sufficing. 1 Another simple and effective appliance for 

 the same purpose, is the Woodward Prism: a small obtuse-angled triangle 

 of glass, whose long face must be brought into immersion-contact with 

 the object-slide by a film of interposed glycerine. Originally devised as a, 

 right-angled prism, it was suited only for the illumination of objects seen 

 under immersion Objectives of widest angular aperture; but by reducing its 

 oblique angles to less than 45, so as to open-out the two equal sides, it. 

 may be adapted to Objectives of much smaller aperture. In using it, 

 the light is made to enter one of the oblique facets perpendicularly to its- 

 surface; and by looking in the like direction through the other side of 

 the prism, the observer can see when the face of the object is best illumi- 

 nated, by the rays reflected on it from the inner surface of that facet. 

 This prism can be made to hang to the under surface of the object-slide 

 by the film of interposed glycerine; but as it is very apt to slip when the- 

 microscope is inclined, and as its full advantage can only be obtained 

 when the object is made to rotate so as to meet the illuminating beam in 



1 For the mode of constructing this Illuminator, see " Journ. of Roy. Microsc. 

 Soc ," Vol. iii. (1880), p. 246. 



