736 THE MICROSCOPE 



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 fastened 

 on 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 an excellent 

 spectrum is obtained in one direction with one reagent, 

 and in the other with another, without further trouble. 



" The scale of measurement consists of two small Mcol'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 45° to the plane of polarisation, 

 though no difference can be seen in the light with the 

 naked eye, the spectrum is entirely changed. The light is 

 still white, but it is made up of alternate black and 

 coloured bands, evenly distributed over the whole spec- 

 trum. The number of these depends on the thickness of 

 the depolarising plate, so that we may have, if we please, 

 almost innumerable fine black lines, or fewer broader 

 bands, black in the centre and shaded off at each side. 



