184 SOME POINTS CONCERNING SHARPNESS IN 



The mirror reflecting arrangement provided with the camera 

 proved to be of value, especially at long exposures, as it provides 

 the possibility of a control of the proper focussing during a long 

 exposure, without having any disturbing effect. 



3. Influence of vibrations and its avoidance. 



Even at short exposures with the arc lamp the sensitiveness 

 for vibrations is very undesirable. The whole instrument being very 

 rigidly constructed, the cause of this sensitiveness was by no means 

 obvious. After a detailed examination, it was found that the 

 comparatively great mass of the tube-carrying upright, with the 

 two tubes (ocular and photographic), illuminator and objective, was 

 responsible for the vibration sensitiveness. The remedy was possible 

 to indicate : the objective is to be mounted by itself, on a special 

 upright with little mass, and must have no direct connection with 

 the tubes; if this be the case, then the inevitable vibrations of the 

 tubes will be of no direct influence on the distance between, objective 

 and specimen — which is the most sensitive point as regards sharp- 

 ness. A slight disadvantage introduced by this modification is that 

 the distance between the illuminator and the objective will be sub- 

 jected to small changes; these, however, seem to be of little con- 

 sequence in comparison with the considerable increase in insensi- 

 bility to vibrations which probably will result. Of course, even in 

 works laboratories it is important to be able to produce good high 

 magnification photographs without too much trouble. 



In this connection the following may be added. 



If the ground of the laboratory is not sufficiently free from 

 disturbance it is necessary to mount the apparatus on some vibra- 

 tion-damping device. Now, it has been found from investigations 

 executed in this laboratory by I. Malmborg* that the simplest thing 

 is to mount the instrument on a solid plate, resting on a thick 

 layer of felt; this, however, must not, as is ordinarily the case, be 

 used in a dry condition, but moistened with a viscous liquid, such 

 as vaseline. The energy of the disturbances is then absorbed 

 through the forced motion of the liquid in the interstices of the 

 felt. This method has been used with great advantage.! 



4. Disposition of diaphragms. 



In the metallographic microscope the cutting off of side-rays by 

 diaphragms is well known for several reasons to be of great impor- 

 tance. As a general principle it can be said that the beam of 

 light is to be reduced as much as possible without interfering with 

 the intensity and uniform distribution of the light, or with the 

 necessary extension of the image. 



Fig. 3 gives diagrammatically the illuminating arrangement. In 

 order to work properly, the image of the source of light — as which 

 the opening of the diaphragm B is to be considered — must fall on, 

 or at least near, the illuminator P, and the image of the iris 

 diaphragm I must fall on the surface of the specimen T. The 

 first item, brought about by the lenses L and F, is necessary in 

 order to be able to use the whole of the light power available, and 



♦ Ann. d. Physik {4), Vol. 44, p. 337, 1914. 

 tBenedicks, /. Iron and Steel Inst. 1Q14. I, p 407 (424). 



I 



