^ 837 ) 



XL. — Immersion Stage Illuminator. 

 By John Mayall, jun., F.E.M.S. 



iRead 12th Novemher, 1879.) 



The illuminating device here described was obviously suggested by 

 Professor Abbe's Apertometer disk. 



It is evident that every apparatus fit for observing the aperture 

 of a high-angled object-glass must likewise be capable of being 

 used for illuminating the marginal zone of such an objective. 



The apertometer-disk itself was found to require modifications 

 as to size, shape, &c., to render it more practical as an illuminator, 

 especially for lamp-light. The modifications here detailed have 

 been designed specially with a view to using the illuminator on the 

 Continental stands provided with concentric rotating stage, many of 

 which do not conveniently admit the use of very obliquely incident 

 light from beneath. 



Diagram f scale. 



The plate of glass A is held on to the foot-plate C by means 

 of vertical spring clips from which it can be removed for cleaning ; 

 the back edge is cut to an angle of 45 ^ as in the apertometer-disk, 

 for total reflection of illuminating rays ; the peripheral margin, 

 admitting the rays, is ground spherically instead of cylindrically, 

 which adds to the illumination ; it is supported by a brass semi- 

 cylinder d which forms part of the foot-plate C. A brass object- 

 plate D, carrying clips for the object-slide, is made to rotate on the 

 upper surface of A round the circular glass plate a which is slightly 

 below the surface of D and ground conical and cemented on to A 

 and acts as a pivot to D, smooth rotation being obtained by means 

 of a film of glycerine interposed. 



The illuminator is to be secured firmly on the rotating stage by 

 clamps on the projecting edges of the foot-plate after the centre of 

 a has been centred with the optical system. The lamp and 

 condensing lens to be suitably adjusted so that the reflected image 



