1867.] Physics. 429 



which is intended ta slip into the eye-end of a microscope, instead 

 of the eye-piece. 



The instrument contains a series of prisms arranged for viewing 

 the spectrum by direct vision, and there is an arrangement for the 

 purpose of obtaining a supplementary spectrum from any object 

 whose spectrum it is desired to compare with that of the object 

 placed on the stage of the microscope ; which object may be either 

 a solution of permanganate of potash in a small sealed tube, a 

 cobalt-blue glass, or anything else which will furnish a standard 

 spectrum for comparison. Milled heads with screw motions enable 

 the observer to adjust the focus of the different parts of the spec- 

 trum and to open and shut the slit vertically and horizontally. 

 Powers of from half-an-inch to 1-2 Oth may be employed, and by 

 using a binocular microscope the object may be brought into the 

 field and examined in the ordinary way through one tube, whilst its 

 spectrum may be observed and compared with that of a standard 

 light by means of the other tube. The object may be illuminated 

 either by transmitted or reflected light, and any of the ordinary 

 accessories may be used for this purpose, such as achromatic con- 

 denser, side reflector, Lieberkuhn, &o. Mr. Sorby proposes the 

 use of a standard interference spectrum, to be used as a scale in all 

 descriptions of spectra, as seen by the spectrum microscope. The 

 scale is produced by a plate of quartz * 043-inch thick, cut parallel 

 to the principal axis of the crystal, and placed between two Nicol's 

 prisms. By this means the whole visible space is divided by dark 

 bands into twelve regular divisions, having in all parts the same 

 relation to the physical properties of light. These are counted from 

 the red end towards the blue, their centres being reckoned as 1, 2, 

 3, &c, and the thickness of the plate is so adjusted that the 

 sodium line exactly corresponds to 3^-. The intensity of the 

 absorption is expressed by the following types : — 



Not at all shaded (blank space) 



Very slightly shaded . . . (dots with wide spaces) 



Decidedly shaded . . . (dots closer together) 



More shaded . . . (very close dots) 



Strongly shaded, but so that a 



trace of colour is still seen (three hyphens close) 



Still darker — /single dash) 



Nearly black (double dash) 



Except when specially requisite, only the symbols . . ., — , 

 — , are employed for the sake of simplicity, and then as signs of 

 the relative rather than of the absolute amount of absorption, and 

 it is assumed that there is a gradual shading-off from one tint to 

 the other, unless the contrary is expressed. This is done by means 

 of a small vertical line over the figure, which shows that there is a 

 well-marked division between them. Definite narrow absorption- 



