200 Original Articles. [April, 



ther, are made quite straight and bevelled, so that by putting a small 

 piece of thin writing paper at each end, pressing down the upper 

 plate, and fixing it with the screws, an opening is left between the 

 plates five inches long and about ^-g^th of an inch wide. The lower 

 plate carries a small arm, fitting into a tube attached to a stand 

 (shown by the dotted lines), such as is employed to hold a bull's-eye 

 condenser, so that the slit is always horizontal, and that we may vary 

 its height from the bottom of the stand, and the inclination of the 

 face of the plates. Then, since it is often requisite to hold different 

 sorts of objects in front of the slit, a piece of brass, to act as a stop, 

 carrying a spring, is fixed on one side, as shown at (c), so that a square 

 piece of glass can be held in the same position, or moved up and down 

 directly in front 'of the slit. In the figure the plates of brass are 

 shown by the black portion (b c), the glass by the unshaded part on 

 the left side below (b), whilst the shaded part at a represents a small 

 narrow tube into which any liquid may be put for examination. Large 

 crystals or other objects, mounted on plates of glass, may also be held 

 in the same manner. In some cases it is desirable to examine solu- 

 tions held in test-tubes. For this purpose two pieces of wood are 

 fixed on the other side of the brass plates, shown in section at (d), 

 extending the whole length, and even projecting beyond them at each 

 end. They are made in the V form shown in the figure, so that, 

 whether the tube be small or larger, its centre may be directly in front 

 of the slit ; and wood is better than brass when it is desirable to 

 examine hot solutions. Towards each end is a spring, so that two 

 tubes may be held, and the spectra of the liquids they contain com- 

 pared. Of course all parts of these fittings must be a good, dead 

 black. I have covered the brass with black silk, which cannot 

 be scratched by the glasses, fastening it down with liquid Indian 

 rubber. 



Since it is often desirable to have a very narrow spectrum, a prism 

 of crown-glass is better than one of flint-glass ; and, when a broader 

 spectrum is required, it can be readily obtained by using a condenser 

 of longer, or an object-glass of shorter, focal length. The prism 

 should be fixed in the position shown at e, so that the light may enter 

 at a e, and pass off at e g, at about equal angles. With crown-glass the 

 front face (e) should therefore be inclined at about 10° to the axis of 

 the microscope. The prism might be fitted just within the tube of 

 the condenser ; but I have fixed it as shown, so that a large Nicol's 

 prism may be inserted above it, to be used as described in the sequel. 

 For some purposes day-light must be used, especially when it is desir- 

 able to have the blue end of the spectrum well seen ; but very often 

 lamp-light is better, for Fraunhofer's lines do not interfere with the 

 observations, the quality of the light is more uniform, and, the lamp 

 being placed at about the same height as the prism, the requisite in- 

 clination of the microscope is then just that which is most convenient 

 for the observer. In using a lamp, a large bull's-eye condenser 

 should be placed between it and the slit, so as to give a broad or 

 narrower image, according to circumstances ; and by throwing the 

 image of the edge of the flame on the object, a larger amount of blue 



