690 SUMMARY OF CURRENT RESEARCHES RELATING TO 



then this distance is the radius of a circle which the available front 

 of the condenser, or other apparatus, must cover, so that light may- 

 enter the objective at the most extreme angle of obliquity. If this 

 distance, which we will call d, be T 3 ^ inch, then the available surface 

 of the condenser must be a circle of at least § inch in diameter. 



" Now, assuming the thickness of the usual object slide to be 

 T V inch, though this is hardly enough, if the angle of aperture of the 

 objective is given, we may find the distance d, for with the thickness 

 of the slide, ^ i nc h, as the cosine, the distance d will be the sine of 

 half the angle of aperture of the objective. If the angle of the 

 aperture of the objective be 120°, or 1'31 N.A. in crown glass of 

 1*52 refractive index, then the distance d would be - 144 inch, 

 which, however, will not introduce any special difficulty in the con- 

 struction of an Abbe condenser, as the connecting, or front, surface of 

 the condenser need not be larger in diameter than ■ 288, or a little 

 over \ inch. But when we come up to 140° crown-glass angle, or 

 1 • 42 N. A., the distance d increases at once to ■ 228 inch, and the 

 connecting surface of the condenser must be at least 0*456, or nearly 

 \ inch in diameter. With so large a front surface, or as it is better 

 expressed, front aperture, the condenser to be fully up to 140° crown 

 glass, will have to be of an equivalent focus of at least \ inch, which 

 with 140° in crown glass, will make the back-aperture 1 • 42 inch, or 

 near ly 7 ^ inch, and in mounting it will be pretty close work to get 

 this inside the substage tube. But let us go a step further and 

 suppose an objective of a crown-glass angle of 160° or 1*49 N.A., 

 which may be expected before long. This angle will increase the 

 distance d to ■ 47 inch, and the diameter of the front aperture of the 

 Abbe condenser must be at least 0*94 or yf inch. Now, as the 

 increase of the angle of aperture of the condenser from 140° to 160° 

 will considerably lessen its working distance, it will have to be con- 

 structed of so much longer equivalent focal distance as to keep the 

 working distance of the slide thickness, of at least 1^ inch focus (sic), 

 and even with this it will be hard to get the required working dis- 

 tance. But a condenser of 1^ inch equivalent focus and 160° crown- 

 glass angle will require a back-aperture of 3 • 98 inches. 



" Attaching this mammoth condenser to a Microscope having a 

 stage, and consequently all the base parts that support it, on the same 

 scale, we should have an instrument of such proportions as would 

 give the appearance of a derrick, rather than that of a Microscope. 



" These examples satisfy us that the Abbe condenser, useful as it is, 

 by no means fully meets all the requirements of oblique illumination, 

 and that practically this illumination cannot very well be made of 

 greater angle than it already has. Hence we have either to find some 

 other suitable means of obtaining still more oblique illumination, or 

 to give up, as useless, the increase of the angle of the object for an 

 increase in performance. 



" So it is wise to consider the solution of this problem of illumina- 

 tion before the further improvement of the objective by the increase of 

 angle. In this direction, I desire to submit for consideration the idea 

 of an oblique light reflector represented in Fig. 132. S represents the 



