104 SUMMARY OF CURRENT RESEARCHES RELATING TO 



further resolution may be procured. "We ought to work with light of 



very small wave-length. Since the wave-length is determined by the 



y 

 quotient A = - the velocity of light divided by the number of vibra- 

 tions, two ways seem to be open in order to obtain a smaller A. We 

 may either decrease the velocity of the light, or increase the number of 

 vibrations. The first can be accomplished by immersing the object in 

 a liquid of high refractive index — glycerin, balsam, salt solution, etc. 

 The method is applied to a certain extent, but does not carry us much 

 further. The second method illuminates the object, not with ordinary 

 white light, but with violet vibrations of higher frequency. It was 

 first proposed by Amici, and is also used. But the intensity of the 

 violet light is very feeble, or, rather, the eye is not very sensitive to 

 violet rays. In photomicrography the second objection does not count, 

 but the feeble intensity remains a drawback. A. Kohler, of Jena, has 

 therefore tried ultra-violet light, notably the rays given out by electric 

 sparks passing between cadmium electrodes. These rays, of wave-length 

 275 p fj., have a high intensity. Dr. Kohler described his new camera- 

 microscope, which has been constructed by the Zeiss Glas Werke, of 

 Jena, before the Breslau meeting of the Naturforscher-Versammlung. 

 The lenses of this Microscope are made of crystal and of fused quartz ; 

 they need only be corrected for spherical aberration, because no chro- 

 matic aberration has to be guarded against when monochromatic light 

 is used. As the ultra-violet light is invisible, however, an artificial eye 

 has to be combined with the Microscope for focussing and adjusting. 

 This artificial eye consists of optical parts made of crystal, and of a 

 retina made of fluorescent glass, which responds to ultra-violet rays. 

 The observer examines through a lens the image thrown on this artificial 

 retina. The instrument can, indeed, also be used for subjective vision 

 by ultra-violet rays, and for this purpose magnesium light, of wave- 

 length 280 /a /*, is still more suitable than the cadmium light. But the 

 fluorescent light is injurious to the eye, and the finest detail can only 

 be studied by photography. Yet the fluorescence helps in bringing out 

 further detail. Dr. Kohler also immerses his specimens — so far, mostly 

 organic tissues — in a mixture of glycerin and water, or in salt solution, 

 of which physiologists make large use. The ultra-violet rays at once 

 show differences in the structure, which, hitherto, staining had alone 

 revealed. Thus the horny portions of the epidermis, the membranes 

 of plant cells, and other parts, are more or less impermeable to ultra- 

 violet rays, so that other advantages are realised in addition to the 

 increased resolution. It would not be surprising if ultra-violet illu- 

 mination should also render good service in metallography. 



Three-Colour Photography.* — Chapman Jones gives the following 

 resume of two processes of colour photography. 



Kdniff^s Three-Colour Process.- — This process, only recently pub- 

 lished, has attracted a good deal of attention, and deservedly so, for it 

 not only illustrates a new principle as applied to the purpose of colour 

 photography, but has been worked' out by its author to a successful 



* Knowledge, i. (1904) pp. 285-6. 



