524 



SUMMARY OF CUREENT RESEARCHES RELATING TO 



Fig. ]24. 



some lateral liglit is always allowed to pass up beyond the field aud 

 thus produce fog. 



Hyde's Illuminator or Oblique Immersion Condenser.* — The 

 apparatus here shown (Fig. 124) is one of the numerous devices that 

 have been constructed specially for use with 

 immersion objectives having apertures greater 

 than correspond to 180° in air, i. e. greater than 

 the "numerical aperture" 1 ( = double the critical 

 angle between glass of mean index 1 ' 52 and air, 

 or 82° nearly). 



It consists of a right-angled prism having 

 a plano-convex lens cemented on the long face, 

 the whole mounted on a metal upright attached 

 to an ordinary substage fitting. The lens is 

 calculated to condense parallel rays to a focus 

 on a balsam-mounted object through a slide of 

 average thickness, with which the illuminator is 

 suitably placed in immersion contact. In this 

 case the ray jiassing through the axis of the 

 lens without refraction will be incident upon the object at an obliquity 

 of 45° to the optic axis of the Microscope, as shown for the central 

 ray in the diagram of the action (Fig. 125). 



A is the front lens of an immersion objective in fluid contact with 



the cover-^'lass ; the 

 Fig. 125. object in bi Isam ; P a 



-L right-angled ;;;~^ism in 



immersion contact with 

 the base of the slide ; L 

 a lens designed to focus 

 the illuminating rays on 

 the object O. 



For the oblique illu- 

 mination, as figured, the 

 apparatus must be placed 

 out of the axis of the 

 Microscope. With this 

 obliquity any objective 

 of less aperture than 90° 

 in glass would give a 

 dark field. If placed 

 nearer the axis it is 

 evident that less oblique rays could be used, as the lower half of the 

 lens and prism would then come into action. 



As originally made, the device was mounted in a brass plate 

 fitting into the stage opening, but it was found more convenient when 

 placed in the substage with means of varying the degree of eccen- 

 tricity, upon which depends much of its power as an oblique 

 illuminator. 



* See tl)is Jourual, ii. (1879) p. 31. 



