434 Mr. H. Sorby on Analysis of Animal and Vegetable [Apr. 11^ 



half the slit, and with the acute angle parallel to and just passing be- 

 yond it. This gives an admirable result, the only defect being that, 

 when the spectra are in focus, their line of j miction is some distance within 

 it ; and therefore to correct this I use a cylindrical lens of about two feet 

 focal length, with its axis in the line of the slit, which can easily be fixed 

 at such a distance between the slit and the prisms, as to bring the spectra 

 and their line of contact to the same focus. In front of the slit, close to 

 the small rectangular prism, is a stop with a circular opening, to shut 

 out lateral light, and a small achromatic lens of about half an inch focal 

 length, which gives a better field, and counteracts the effect of the concave 

 surface of the liquid in the tubes used in the experiments, if they are not 

 quite full. These are cut from barometer-tubes, having an internal diameter 

 of about one-seventh of an inch, and an external diameter of about three- 

 sevenths of an inch. They are made half an inch long, ground flat at each 

 end, and fixed with Canada balsam on slips of glass two inches long and about 

 six- tenths of an inch wide, so that the centre of the tube is about one-fourth 

 of an inch from one edge. By this arrangement the liquid may be examined 

 through the length of the tube by laying the slip of glass flat on the stage 

 of the microscope, or through the side of the tube, by placing the slip 

 vertical and the tube horizontal. Cells of this size can be turned upside 

 down and deposits removed without any liquid being lost ; and the upper 

 surface of the liquid is sufficiently flat, even when inclined at a considerable 

 angle. If requisite, small bits of thin glass can be laid on the top, which 

 are held on by capillary attraction, or may be fastened with gold-size, if it 

 be desirable to keep the solution for a longer time. When the depth of 

 colour is too great in the line of the length of the cell, we can at once see 

 what would be the effect of about one-fourth of the colour by turning it 

 sideways ; and thus we can save much time, and quickly ascertain what 

 strength of solution would give the best result. Very frequently we ob- 

 tain an excellent spectrum in one direction with one reagent, and in the 

 other with another, without further trouble. I have constructed a small 

 stage, too complicated to describe in writing, which enables me at once to 

 examine solutions in two such tubes, either endways or sideways, and com- 

 pare their spectra side by side, or to use test-tubes, or to fix the small 

 apparatus which I have contrived for accurately measuring the spectra. 

 This is of such great importance in these inquiries that I must describe it 

 in some detail. 



3. Scale of Measurement. 

 It consists of two small Nicol's prisms, and an intermediate plate of 

 quartz. If white light, passing through two such prisms, without the 

 plate of quartz, be examined with the spectrum-microscope, it of course 

 gives an ordinary continuous spectrum; but if we place between the 

 prisms a thick plate of quartz or selenite, with its axis at 4,5° to the plane 

 of polarization, though no difference can be seen in the Hght with the naked 

 eye, the spectrum is entirely changed. The light is still white, but it is 



