ZOOLOGY AND BOTANi% MICEOSCOPY, ETC. 397 



in diameter, and althongli the lenses are not perfectly acTiromatic, yet 

 it is not too strongly coloured at the margin by chromatic aberration. 



Below the doublet another piece of apparatus can be screwed with 

 either two springs, or better a double fork h, for holding the coloured 

 solutions or glasses for producing monochromatic images, also the 

 media for the absorption of the dark heat-rays. If it be required to 

 have additional vessels for the absorption of heat or to employ 

 different absorption media at the same time, others can easily be 

 fastened under the forks by indiarubber rings, the height of the 

 stage c above the mirror giving sufficient space for four or five. It 

 is not advisable to fix them above the lenses upon the stage c, because 

 while the warmth beneath the lenses extends uniformly through the 

 whole of the fluid, there is above them a very hot cone of rays, which 

 strongly heats a small j)ortion of the absorption liquid, and with 

 liquids such as iodine in bisulphide of carbon explosions may easily 

 take place. Indeed, it is in this case necessary, instead of the Desaga 

 bottles (at first exclusively employed by Professor Pringsheim), to 

 use glass boxes for holding the absorption fluids, of greater width 

 than the aperture of the doublet. Tor this purpose round, well- 

 polished glass rings can be employed, 10 mm. deep, closed on either 

 side by flat glass plates, held together by strong indiarubber rings. 

 If these are carefully closed, all aqueous solutions can be kept in 

 them for months without evaporating to any considerable extent, par- 

 ticularly as a stratum of small crystals speedily forms at the edge, and 

 thus makes them still more air-tight. Solutions of bisulphide of 

 carbon must often be renewed, because they evaporate, even when 

 most tightly closed. 



After many experiments, the following have been proved to be the 

 best absorption fluids : — For the absorption of red-yellow, a solution 

 of ammonio-oxide of copper ; for the blue and red ends of the spectrum, 

 solutions of chloride of copper, obtained by the evaporation of a 

 saturated solution of the salt, according to the intensity of the colour 

 and the extent of the absorption ; for the green-violet, a solution of 

 bichromate of potassium (KaCraOy) ; and for the orange-violet a 

 solution of iodine in bisulphide of cai'bon or iodine in iodide of 

 potassium. As far as can be at present ascertained, solutions of 

 organic pigments or of aniline colours are unsuitable, at least they 

 possess no superiority over the above solutions. Coloured glass 

 plates may be used, if perfectly uniform. Of course, the value of all 

 media for absorption must first be tested in the spectroscope. Water 

 or a concentrated solution of alum can be used for the absorption of 

 the dark heat-rays. 



Above the fixed lower stage is the movable stage o, moved by the 

 screw t^. It is pierced in the centre, and serves to carry the slide, the 

 gas chambers, &c. By means of the screw, the object can be brought 

 into the plane of the sun image formed by the lenses, or immersed in 

 it if necessary. The screw t^, as well as t, which moves the micro- 

 scope-tube, works on a triangular bar z. The screw t gives the coarse 

 focussing, after the object on the stage has been adjusted by means 

 of <\ whilst the micrometer-screw m gives the necessary tine focussing 



