28 Transactions of the Society. 



all develop the markings on difficult tests. For if we continue to 

 increase the angle of the mirror [he refers to diagram] the object 

 first acquires a pearly appearance, and is afterwards seen in a dark 

 field known as ' Keade's back-ground [black-ground ?] illumina- 

 tion' .... but the markings have again become indistinct or 

 disappear altogether, showing that it is needful to allow a small 

 portion of the light from the source of illumination to pass into the 

 object-glass, and through the object, that the striae may either be 

 rendered more visible by the rays that they intercept, or that the 

 field shall be partly luminous." 



Withiu the last few years the apertures of objectives have been 

 so considerably extended by means of the immersion system, that, 

 in order to utilize their fullest power, it has been found neces- 

 sary to use an immersion system of illumination. By these means 

 we obtain direct rays (i. e. rays other than those merely deflected 

 by the object) from the illuminator at greater inclination than the 

 critical angle, which certain of these immersions will transmit, pro- 

 ducing a luminous field. 



When the object is in balsam, and the base of the slide plane 

 and in air, no rays can reach it from beneath at an obliquity greater 

 than the limiting angle for balsam. In order that direct rays may 

 enter the balsam beyond the inclination of 41°, we must have 

 recourse to an immersion condenser, or something equivalent. 



But it must not be supposed that the limiting angle at which 

 rays could be admitted into balsam from beneath, through a flat 

 plate of glass, imposes the same limit to the angle up to which an 

 immersion objective could collect image-forming rays, supposing 

 them to have got into the balsam, — which assumes that the image- 

 rays above the object are limited by the angle of the direct illumi- 

 nating rays from beneath. This erroneous view has had some 

 currency, and may be thus stated : — Because the object in balsam 

 cannot receive light from beneath beyond the hmiting angle 

 for balsam, unless wo have an immersion system of illumination 

 (supposing the base of the slide plane and in air), therefore there 

 are no rays from the object beyond that limit to be transmitted by 

 the immersion objective, however great its aperture ; the question 

 arising — '" Where can such rays come from ? " 



It is evideut that, independently of the angular direction of the 

 illuminating rays, if there be an object in the field capable of 

 scattering (and not merely intercepting) light, it is seen luminous 

 by scattered rays. Kegarded then as a self-luminous object, rays 

 are nascent therefrom and scattered equally in all directions, and 

 therefore at greater inclination than 41°. There is no theoretical 

 difficulty in their reaching the second surface of the front lens of 

 an immersion of suitable form, and in their being transmitted. 

 They cannot, however, take part in the formation of the image by 



