202 Transactions of the Society. 



In using the objectives for photograpliy, where the image must 

 be at a considerable distance, unless an ordinary low eye-piece is 

 used to photograph with, an auxiliary lens becomes requisite, which 

 will remove the image to the required distance, without altering 

 the course of the rays in the objective itself. For this purpose 

 a concave lens of suitable focal length may be inserted close behind 

 the objective in the same way as a short-sighted person uses concave 

 spectacles to move the plane of distinct vision to a greater distance ; 

 a concave lens of relatively corresponding shorter focal length may 

 also be interposed at a greater distance from the objective, in order 

 to produce a moderate amplification (two or three times) of the image, 

 and at the same time a decrease in the requisite distance of the 

 plate. The position of the auxiliary lens in this case must of course 

 be so regulated, by computation, that the cones of rays emerging 

 from the objective converge towards the same j)lane as in ordinary 

 observation. 



A second point which must not be lost sight of in using these 

 objectives— and in fact any objective the numerical aperture of 

 which considerably exceeds the value 1 — relates to the conditions 

 which the illuminating apj)aratus must satisfy, in order that the 

 whole angular aperture may be utilized with oblique illumination. 



With a numerical aperture of 1 • 25 an incident ray, if it is to 

 reach the external zone of the objective, must, when it impinges on 

 the object, be incident towards the axis of the Microscope at an 

 angle of about 56°. Eays with this inclination cannot of course 

 be transmitted to the objective out of air through a flat surface 

 perpendicular to the axis, such as the lower surface of the glass 

 slide. An incident ray reaching this surface from below would not, 

 after entering the glass, be inclined towards the axis more than 

 about 42° ; and with the ordinary illuminating mirror even this 

 obliquity could never be attained, apart from the great loss of light 

 by reflection, which would greatly detract from the effect. In 

 order therefore to utilize the maximum degree of oblique illumina- 

 tion, which an objective of such large aperture will admit — of 

 course with objects which do not lie in air — and to bring out the 

 full defining power of the objective, an illuminating apparatus is 

 necessary, which not only gives a cone of rays of equal aperture 

 with the objective, but which at the same time admits of a fluid 

 connection with the under side of the slide. One immersion c in- 

 denser amongst others which fulfils these conditions, is the illumi- 

 nating apparatus described by me * some years ago, the system of 

 lenses in which (corresponding with the angle of aperture of the older 

 immersion objectives of Zeiss) possesses a " numerical aperture " of 

 over 1 • 1 for its upper focus, and in the construction of which the 

 * Max Schultzc's ' Aicliiv f. IMikr. Anat.,' ix. 496. 



