462 SUMMARY OF CURRENT RESEARCHES RELATING TO 



may be mentioned, with which very fine results were obtained. If, for 

 instance, one is required to take a negative of some grains of sand to 

 show the character of any particular variety, the procedure may be as 

 follows : Take an ordinary glass slip 3 in. by 1 in., give it a few strokes- 

 with virgin wax (white wax), hold the slip over a Bunsen burner, or 

 spirit lamp, until the wax melts, which may then be spread with the 

 finger, then sprinkle the sand over the melted wax, to which it adheres. 

 The wax will cool at once. The slide is placed on the Microscope stage, 

 with a piece of dead black paper behind it, and after focussing by 

 means of a gas-jet or lamp, the illumination is made by burning a few 

 inches of magnesium ribbon held behind the objective, and gently 

 waved about. The objective used in such a case may be a 70 mm. by 

 Zeiss, and a camera extension of about 18 in. A great number of 

 rock sections, to enable them to be photographed well, require the use 

 of polarised light to differentiate their structure, and many also as well 

 the use of a depolarising selenite. Some of them show best for photo- 

 graphic purposes when examined with tbe crossed nicols only without 

 the selenite, while others again absolutely require the selenite to reveal 

 the structure properly. Agate and the various forms of felspar may 

 be mentioned as rendering this illumination necessary ; while, on the 

 other hand, the selenite may not be used with many of the forms of 

 granite, as the crystals of granite show so much colour that only 

 the crossed nicols are required. No absolute rule can be laid down as 

 to the mode of procedure either as regards the illumination required or 

 the use of polarised light. Every one must just exercise his own dis- 

 cretion and skill in such matters. When a considerable experience with 

 the Microscope is possessed by the operator no difficulty will be found 

 in judging what is the best mode of operating to obtain the best 

 results. One thing is essential above all others for success in this 

 work, and this undoubtedly is that the sections must be thin. There 

 is not much difficulty nowadays in getting thin sections compared to 

 those which could be obtained some years ago. Another point is that 

 the objectives used must be corrected for the chemical focus. It will not 

 do to attempt this work with any ordinary micro-objectives. Even 

 with the low powers which are, except in certain cases, only required, 

 the results obtained must be sharp and clear. They have to be so, as, 

 when projected by the lantern, defects become so very apparent. The 

 illumination used by the writer is invariably magnesium ribbon. The 

 image is first focussed by an ordinary lamp, which is then removed, and 

 a small piece of brass tube, about ■{*■$ in. in diameter, having been pre- 

 viously fixed in a shutter, and placed exactly in line with the optical 

 axis of the Microscope, the magnesium ribbon is pushed through the 

 tube and ignited. In this way the exposures are so short that little 

 time is lost. No instructions, however, will render experience useless. 

 It is only by long practice that any one can hope to succeed in any 

 department of photography. 



<J rosbie, F. — Directions for Photomicrography. Lancet, 1903, p. 233. 



D'Abct Poweb, H. — Laboratory Photography. 



Journ. Applied Micr., vi. (1903) p. 2282. 



Elliott, L. B.— Ditto. Ibid., p. 2230. 



FracHEn, H. — Mikrophotogramme von Inulitsphariten und Starkekbrnern. 



Ber. d. Deutsch. Bo'an. Geeelhch., xxi. (1003) p 107. 



