In examining crystals or other small objects, a small cardboard diaphragm should be placed beneath them; and when examining the spectra of liquids in cells, slip a small cap with a perforation of 1/10-inch in diameter over the tube containing the 1/2-inch or 2-inch objective. Substances which give absorption bands or lines in the red are best seen by artificial light, while those which show bands in the violet are better seen by daylight. By following rules of the kind we are less likely to mix the bands of the absorption spectrum with the Fräunhofer lines. For example, if the edge of a band happens to coincide with a Fräunhofer line, the observer is apt to imagine that the band is better defined and more abruptly shaded on one side than it really is.
_Cells and Tubes._--These are either supplied ready-made by the optician, or can be formed out of small pieces of barometer tubing, with the edges ground down and cemented on ordinary glass slips. In Fig. 198 is seen the several kinds of cells and tubes usually employed, while the little flat tubes commonly in use as bouquet holders will be found of use, with the side stage reflecting spectrum as comparison tubes; being of different diameters they allow of two or more depths of colour in the fluid intended for examination.
In the case of many other fluids the sloping form of cell (Fig. 198) will be useful, as different shades of fluids can be examined without removal from the stage of the microscope. The deeper cells are cut from a piece of barometer tubing of about half to an inch long, one end being cemented to a piece of flatted glass, and the other covered over temporarily or permanently with a thin piece of glass on the top, held in its place by capillary attraction, thus admitting of the tube being turned upside down.
_Re-agents required._--A diluted solution of ammonia, citric acid, double tartrate of potash and soda (the last being used to prevent the precipitation of oxide of iron), and the double sulphate of the protoxide of iron and ammonia (employed to deoxidise blood, etc.). In some special cases, dilute hydrochloric acid, purified boric acid, and sulphate of soda are required.
The character of stains of blood varies with age and with the nature of the substance with which it happens to be combined. This is important to remember in connection with _Jurisprudence_, when the micro-spectroscope is brought into use for the detection of blood stains. The spectrum used in important cases of the kind should have a compound prism, with enough, but not too great dispersive power, otherwise the bands become, as it were, diluted, and less distinct.