ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



607 



Fig. 132. 



necessary to prepare one flat surface in the specimen. The eye-j)iece is 

 placed horizontally, and receives the image by the reflection of a totally 

 reflecting prism E placed under the objective. The examination can, 

 therefore, be conveniently carried on while the operator is sitting in 

 front of a table. 



The adjustment is done through a screw collar surrounding the 

 objective and receiving the support B. The surface 

 under examination must be absolutely perpendicular 

 to the axis of the objective. The supports which 

 rest upon three points have a very uniform thickness, 

 so that the preparation is in the proper position. In 

 a Microscope for the examination of metal the illu- 

 minating arrangement requires the most delicate regu- 

 lation, and it is generally accomplished tentatively 

 by adjusting a number of movable pieces. But in this 

 instrument only two adjustments are required, the 

 proper opening and proper position of a single dia- 

 phragm M placed in such a way that its opening regu- 

 lates the angle of the beam of light which falls upon 

 the preparation. This angle must vary with the 

 nature and the quality of the objective. The greater 

 the angle, the greater the effect of the spherical aber- 

 ration ; and, on the contrary, the smaller the angle, 

 the greater the chromatic aberration. The best defi- 

 nition is obtained with a certain mean angle which 

 must be found tentatively in each case. The position 

 of the diaphragm M regulates the mean inclination 

 of the beam of light falling upon the preparation. 

 To obtain the greatest clearness, its direction should 

 be nearly vertical ; but in order to reduce the amount 

 of light reflected by the lenses, and sent back to the 

 eye, it is necessary to give a more inclined direction 

 to the beam of light. If the inclination is what it 

 should be, most of the light reflected in this way is 

 stopped by the illuminating prism itself. This dia- 

 phiagm M is placed at the principal focus of the 

 complex optical system made of the objective D, the 

 illuminating prism J, and the lens L. To ascertain 

 its proper position, the image of a luminous point 

 placed at a distance of two or three metres above the 

 objective is located by means of a magnifying glass. 

 This determination is done once for all for each ob- 

 jective, and the corresponding position, in each case, 

 of the draw-tube carrying the diaphragm is noted. As a source of 

 light, if the object is to be viewed only with the eye, a Welsbach lamp 

 is the most convenient. For photomicrography, the whole of the photo- 

 graphic outfit is placed vertically below the objective. If white light is 

 to be used, a Welsbach lamp, an acetylene flame, or an oxyhydrogen 

 light are very satisfactory. But it is preferable to use a monochro- 

 matic source of light, because so-called achromatic and apochromatic 

 objectives are never completely free from chromatic aberration. A 



