1882.] 



MICKOSCOPICAL JOUEISAL. 



BY 



face, 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 

 one half-inch, which, with 140° in 

 crown-glass, will make the back-aper- 

 ture 1.42-inch, or near i^^ of an 

 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 0.47 -inch, and the di- 

 ameter of the front aperture of the 

 Abbe condenser must be at least 

 0.94 or -^-inch. Now, as the in- 

 crease of the angle of aperture of the 

 condenser from 140** to 160° will con- 



proportions, as would give the ap- 

 pearance 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 require- 

 ments of oblique illumination, and 

 that practically this illumination can 

 not 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 objective for an increase in 

 performance. 



So, it is wise to consider the solu- 

 tion of this problem of illumination 

 before the further improvement of 

 the objective by the increase of an- 



Fi&. 29. 



siderably lessen its working distance, 

 it will have to be constructed of so 

 much longer equivalent focal distance 

 as to keep the working distance of 

 the slide thickness, of at least xyz- 

 inch focus, and even with this it will 

 be hard to get the required working 

 distance. But a condenser of i^- 

 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 

 would have an instrument of such 



* The radius of the actual (back) aperture 

 of an objective is to the equivalent focal dis- 

 tance of the latter, as the sine of half the 

 angle of aperture of the objective multiplied 

 by the refractive index of the medium to 

 which the angle of aperture relates, is to the 

 radius of this angle. 



gle. In this direction, I desire to 

 submit for consideration the idea of 

 an oblique light reflector represented 

 in fig. 29. S represents the object- 

 slide ; r the proposed reflector. It is 

 a section of a sphere. The upper, 

 plane surface is to be brought in 

 contact with the slide by means of a 

 suitable fluid in the usual way. The 

 under-surface is concave. The dot- 

 ted lines show the direction of the 

 light, which undergoes an inner total 

 reflection at the surface c, c. 



Perhaps this reflector will answer 

 for the next limited period ; and, 

 when even this shall prove to he in- 

 sufficient, I propose to mount the ob- 

 ject on the plane surface of this re- 

 flector. In this way, the theoretical 

 limit would be reached, and opticians 

 can go on constructing objectives 

 that will take and utilize the oblique 

 light of this reflector. 



