186 SOME POINTS CONCERNING SHARPNESS IN 



mirror, the resolving power being reduced to one-half in the direc- 

 tion, at right angles to the mirror edge.^ 



This fact is amply borne out by our new comparative experi- 

 ments. Thus, Fig. 4 shows the very best definition to be obtained 

 with a prism illuminator. A comparison with Fig. 1, which was 

 obtained with the plain glass illuminator, gives evidence of the much 

 higher quality of the image obtained in the latter case, thus laying 

 stress on the fundamental condition for obtaining sharp high mag- 

 nification micrographs: the full utilisation of the aperture of the 

 objective. 



7. CoiniKir'iaon heiween prism and metal mirror. 



It is obvious that with reflecting glass prisms — as well the Le 

 Chatelier as the 45° prism — inner reflections cannot be entirely got 

 rid of. On the other hand, with a metal mirror, such undesirable 

 reflections do not occur, and it is to be expected that the contrasts 

 will improve. 



As the result of some direct comparisons, it was actually found 

 that an indisputable, though slight, increase of the contrasts was 

 to be seen on the micrographs obtained with the metal mirror. Thus, 

 a metal mirror illuminator may be of some use whenever particularly 

 strong contrasts are desired. 



8. Influence of the thickness of the jilain glass. 



The glass slide provided with the microscope used was 0.45 mm. 

 thick. It may be questioned whether this thickness, on account 

 of the astigmatism introduced, is not too high. On using a very 

 thin glass, 0.10 mm., as a matter of fact, a slight improvement of 

 the sharpness occurred; this, however, was so insignificant that 

 practically the use of the thicker glass must be considered to be 

 quite justified. 



If one is at liberty to choose, a thinner glass, of course, should 

 be preferred to a thicker. 



9. Astigmatism introduced hy the right angle reflecting prism. 



It seems by no means excluded, that sensible astigmatism could 

 not be introduced by the right angle prism used in the Le Chatelier 

 microscope in order to reflect horizontally the vertical beam of light 

 issuing from the specimen. However, the excellent definition 

 obtained, as in Fig. 1, shows that this undesirable influence of the 

 prism can be entirely neglected. Of course, it is essential that 

 the prism be of a very high optical finish, and carefully adjusted. 



10. Platinised plain glass illuminator. 



As ])ointed out on an earlier occasion, f it might be possible to 

 increase the light intensity obtained with the plain glass illuminator 

 by using a thin silver or platinum coating. Obviously, the thick- 

 ness of the metal layer must not exceed a definite value; otherwise 

 a decrease of intensity will result. 



* See for instance Dr. Ro'Sienhain's An Introduction to the Study 

 of Physical Metallur^yj London, 1Q14, p. 52. 

 t C. Benedicks. I.e. 



