\YITH THE MICROSCOPE. 223 



sizes are -^-incli long, -yth inch in internal, and somewhat under ^ inch 

 in external, diameter. These are ground flat at each end and 

 attached with Canada balsam near one edge of a glass plate, so that 

 they may be examined either end-ways, by laying the plate flat on the 

 stage of the microscope, or side-ways, by leaning the glass against the 

 side of the object-glass. If then the colour is too deep in the line 

 of the length, the tube can be turned, so that it may be examined 

 side-ways, which being equivalent to using |th the quantity, we can 

 easily judge what amount would show the most perfect spectrum. 

 The cells should be either filled level, or covered with a piece of 

 thin glass. If the diameter of these cells be less than \th of an 

 inch, it is difficult to fill and empty them ; and, if much wider 

 than th, the liquid is apt to run out when they are turned over ; but 

 when of about yth wide they are easily filled and not a drop of liquid 

 is lost even when they are turned upside down. Almost all kinds of 

 testing can be carried on in these cells, and they may be easily washed 

 out by means of a small stream of water blown out of an ordinary 

 chemical wash-bottle, pi. XXII, fig. 143. Solid or liquid reagents can 

 easily be added and stirred up by means of a moderately stout plati- 

 num wire, flattened at one end, and turned up square like a small 

 hoe. The great advantage of these cells is that a very small quantity 

 of material is required (which is most important in some investiga- 

 tions) ; but for some purposes ordinary test-tubes are very useful, 

 and especially to place on the stage attached to the eye-piece and 

 compare with objects on the stage of the microscope. These can 

 be examined only at the sides and the colour must, therefore, be 

 diluted so as to show the best spectrum. Wedge-shaped cells like 

 fig. 325, are also useful in order to study the effects of different 

 thicknesses of solutions. Colours which do not materially change 

 on keeping some time may be mounted in tubes, see fig. 326, about 

 |-inch in diameter and 3 inches long, sealed up flat at the bottom, 

 and drawn out capillary at its top, leaving a small opening through 

 which the liquid may be introduced by means of an air pump, 

 afterwards sealed up with the blow pipe. Many mineral salts can 

 thus be kept for an indefinite period; and even many animal and 

 vegetable colours can be kept for a year or more always ready for 

 examination. 



32O. Examination of Blow-pipe Beads and .Solutions in Cells. 

 The coloured beads obtained by ordinary blow-pipe testing, can 

 easily be examined by the spectrum microscope ; and in some cases, 

 give very satisfactory results. Some crystals also are excellent 

 objects, and give striking spectra. It is easy to select such as give 

 the best result, or to cut them wedge-shaped, and thus examine 



