712 SUMMARY OF CUKKENT RESEARCHES RELATING TO 



passing the object /, reach the objective. All these rays must pass 

 the point at h within the diaphragm image. Therefore, by pushing 

 the frame from right to left, when the edge of the plate e approaches 

 the axis, only a narrow strip of light from the diaphragm-image will 



remain, and this will also disappear by 

 YiG. 151. ^ further movement of the frame. At 



the same instant, the field becomes dark, 

 but the rays remain which deviate in the 

 object towards the left — as in Fig. 150 

 the ray i — and the corresponding points 

 appear bright. Spots in the object are 

 thus easily recognized which would 

 otherwise pass unnoticed in consequence 

 of the brightness of the field. Only those rays are effective which are 

 deflected at right angles to the edge of the frame. The apparatus 

 must therefore be so adjusted that the object can be turned round the 

 optic axis, while all else remains immovable. 



The manipulation of the apparatus is as follows : — The frame c c 

 is placed in a central position so that the open space between d and 

 e is in the optic axis, and the Microscope is accurately focused on 

 the object. The latter is then pushed aside, so that there is now an 

 open space in the stage under the objective, and the glass plate d is 

 brought into the axis. The semicircular diaphragm is now so ad- 

 justed that its image appears clearly on the glass, and the straight 

 edge in this image exactly parallel with the edge of the plate e, but 

 turned away from it, so that on moving the frame the convex side is 

 first shut off, and finally only a narrow line of light remains. The 

 adjustment of the diaphragm is effected by sliding it up and down. 

 The position of the tube must not be altered, or else, if it is again 

 adjusted to the object, the image of the diaphragm will no longer lie 

 in a plane with e, which is an absolute necessity. The frame being 

 now so adjusted that the rays can pass through it unhindered, the in- 

 strument is ready for observation. The frame is moved slowly by 

 the screw h till the edge e meets the optic axis and the direct rays 

 a: e cut off. The field is now dark, but all points in the object which 

 have a greater or less refractive power are brightly illuminated on the 

 dark ground. If the frame be moved still more, these rays also dis- 

 appear. The proper moment for observation is, therefore, when all 

 direct light is shut off. 



This apparatus is only suitable for low powers ; with high powers 

 many inconveniences arise. The frame must of necessity be brought 

 quite close to the lenses, for if the whole is to be obscured at once, 

 the frame must be exactly at the place where the image of the 

 diaphragm is formed ; if it is further away, only half of the field is 

 effective. The nearer to the lens, however, the greater is the spherical 

 aberration, because the objectives are properly corrected only for an 

 image distance, equal to the length of the tube ; the image of the 

 diaphragm will not be very sharp, and the rays diverted in the object 

 mingle with the indistinct margins of this image. A further incon- 

 venience arises from the fact that in objectives having a focus of 

 3 mm., the distance between the object and the diaphragm must be so 



