Immersion Illuminators. By J. Mayall, jun. 29 



a dry objective, because they are internally reflected by the cover- 

 glass. These rays must be regarded as important for dehcate 

 markings, as evidenced by comparing the definition we obtain with 

 the highest-angled immersions and dry objectives on a balsamed 

 object with ordinary illumination, — that is to say, when the base of 

 the slide is plane and in air. 



The utilization of the whole of the very large cone of rays that 

 might be condensed on the object by using an immersion illuminator 

 having an aperture equal to that of the objective, in other words, 

 the direct illumination of the whole aperture, is not the problem 

 that has engaged the attention of those who have endeavoured to 

 exhibit the fullest power of the apertures of immersions. It was 

 long ago found that it is not so much mere quantity of light that 

 is required on the object, as diflerence of illumination that can be 

 rendered perceptible by the eye. The more difficult images are seen 

 only as we utilize the extreme marginal aperture of the objective 

 and the more oblique direction of the illuminating pencil. This 

 can only be done practically by excluding all excess of central light. 

 The objects on which the fullest power of the aperture is needed 

 are generally so nearly of the same refractive index as the fluid in 

 which they are immersed, that there is difiiculty in making delicate 

 differences of transparency perceptible. The immersion system of 

 illumination becomes all-important to this end, as, by it, any required 

 degree of intensity of light can be got upon the immersed object at 

 the most favourable obliquity for the aperture of the objective. 



It is found in practice that to obtain the fullest effect on the 

 object, of the extra-ohYio^we rays provided by immersion illumination, 

 the objective must have an aperture capable of transmitting them, so 

 that the field is luminous ; they thus become a practical proof of the 

 extent of the aperture. It follows also, as matter of observation, that 

 up to the angle to which the objective refracts the direct rays from 

 the illuminator to a luminous field, to that angle (or very nearly so) 

 it refracts image-rays from the object ; for we find that increasing 

 the obliquity of the direct illuminating rays so as to approach to 

 the dark-field produces, at the same time, distortion of the image, — 

 showing that both systems of rays traverse the objective together. 



The angle of the direct illuminating rays must not, however, be 

 regarded as an essential condition of the existence of the aperture 

 (as such). It proves the extent of the aperture of the objective by 

 direct transmission ; its effect in rendering visible minute structure 

 is plainly matter of experience, — and experience shows that, so far 

 as apertures have been carried, the gain has been in proportion to 

 their capacity for direct transmission. 



It will be understood that I refer only to objectives in which 

 the corrections have been made to the fullest extent of the aperture ; 

 for it must be agreed that there is no such thing as ajjerture, 



