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SUMMARY OP OUBRBNT EESEABOHES RELATING TO 



been endeavouring for some time to devise a method of comparing 

 unknown opaque minerals with others which have been already 

 determined. For ten years no progress was made in this direction. 



My first attempt was made by means of the camera lucida, which 

 transmits perfectly both the colour and the lustre of opaque minerals ; 

 with the help of this instrument I transfer the image from one 

 Microscope into a second, on the stage of which is a known mineral, 

 and so am able to compare the two. But to prevent the image of the 

 first Microscope from covering that of the second the following pre- 

 cautions must be taken. Into a Hartnack's camera lucida (fig. 94) I 



Fig. 94. 



U'v^-'^A/J 



introduce a diaphragm a, which is placed in the lower part of the tube, 

 so as to cover half the field of view ; a similar diaphragm a, that 

 is, one which also covers half the field, is introduced into the second 

 Microscope, which contains the known mineral. By means of this 

 arrangement of the diaphragms I see in the second Microscope on one 

 side (the left) the image of the mineral to be determined, and on the 

 other side (the right) that of the known mineral. The apparatus, as 

 described above, has always one great fault, that is, that the comparison 

 is made, so to say, between an object and a shadow, for the camera 

 Incida always slightly increases tiie image which it transmits, and 

 consequently diminishes its brightness. I have now, however, found 

 a method of comparing minerals under identical conditions, and I 

 have only mentioned the camera lucida and my first attempt because 

 every one possesses this instrument, and can easily test the method. 



To secure a complete identity between the image of the mineral 

 to be determined and that with which it is compared, I have had a 

 new apparatus constructed, which may be called the Comparison 

 Chamber or Microscopic Comparer (fig. 95), which enables us, as it 

 were, to elongate two Microscopes, and bend them at right angles. 

 At the outer corners of the apparatus are placed totally reflecting 

 prisms or small mirrors, which receive the rays that emerge from the 

 Microscopes and reflect them at right angles. Below the opening, in 



