ACCESSORY APPARATUS. 



109 



-ciple, however, having 

 Immersion Paraboloit 



been more recently taken up by Dr. Edmunds, an 

 specially devised by him for use with immersion 

 Objectives of large aperture, has been constructed by Messrs. Powell & 

 Lealand, with results so satisfactory, that it now ranks among the 

 Accessories most valued by such as habitually work with Objectives of 

 that highest class. 1 



107. Wenham's Reflex Illuminator. Another very ingenious and 

 valuable illuminator for high powers has been devised by Mr. Wenham, 3 

 and constructed by Messrs. Koss. It is composed of a glass cylinder 

 (Fig. -80/ a) half-an-inclrlong, 

 .and four-tenths of an inch in 

 diameter; one side of which, 

 starting from the bottom edge, 

 is worked to a polished face at 

 an angle of 64 with the base. 

 The top of the cylinder is polish- 

 ed flat, whilst its lower surface 

 is convex, being polished to a 

 radius of 4-10ths of an inch; 

 -close beneath this last is set a 

 plano-convex lens of 1J inch 

 focus; and the combination is 

 set eccentrically in a fitting, i i, 

 .adapted to be received into the 

 Sub-stage. The parallel rays, 

 / / /, reflected up into it from 

 the mirror, are made to con- 

 verge, by the convex surfaces at 

 the base of the cylinder, at such 

 .an angle, that if their course 

 were continued through glass 

 they would meet at the point 

 h, above the glass slide c ; but 

 by impinging on the inclined 

 polished surface, they are reflect- 

 ed to the flat segmental top, 

 from which again they would be 

 reflected obliquely downwards 

 so as to meet in the point 

 b, but for its being brought into ' immersion-contact ' with the under 

 .side of the slide. Passing upwards through the slide, they meet in a 

 point, g, a little above its upper surface, in the optic axis of the Micro- 

 scope, to which point the object must be brought; 'and by giving rota- 

 tion either to the object or to the illuminator, it may be illumined from 

 every azimuth. For convenience of centering, a black half-cylinder e, is 

 .so fixed by the side of the cylinder, that if a dot upon its upper surface 

 be brought into the centre of the field of view of a low-power objective, 

 its focus g, will lie in the optic axis. Some skill and practice are 

 required to use this apparatus to advantage, but it will amply repay the 

 trouble of mastering its difficulties. It is best suited to thin flat objects; 

 with those that are thick and irregular, distortion is unavoidable. 



Wenham's Reflex Illuminator. 



1 " Monthly Journ. of Microsc. Sci.," Vol. xviii., p. 78. 

 *Ibid., Vol. vii., p. 239. 



