482 Miscellaneous. 



ed by the mixture of those existing in one portion of the spectrum 

 with that formed from the mixture of those remaining, that white 

 light is produced. The two colours formed by these combinations, 

 each consisting of that colour which the other requires to compose 

 white light, are called, for that reason, complimentary to each other. 

 All we have to do, then, to render a reddish-yellow light white, is to 

 mix with that colour the complimentary one. The colour must vary 

 according to the nature of the light and the quality of the combus- 

 tible : if the light be reddish, a pale green glass, in addition to the 

 blue, will be requisite. The readiest mode of ascertaining the pro- 

 per tint is to fit the polariscope to the microscope ; then to place in 

 the stage some crystallized salts belonging to any other system than 

 the cubic ; next to arrange the analyzer and polarizer so that their 

 planes of polarization are at right angles : by examining thus several 

 crystalline specimens, a jDortion may be always found which is of ex- 

 actly the same colour as that of the flame (which must be found out 

 by comparison) ; by then turning round the analyzer, so that the 

 planes of polarization become parallel, the complimentary blue tint 

 will be found, and a piece of glass of this colour will be the requisite 

 one. 



In applying this principle to the illumination of microscopic ob- 

 jects, I found that by passing the light, in its passage from the can- 

 dle or lamp, through a piece of deep-blue glass, I could render the 

 light, as I had anticipated, perfectly colourless. By this light bodies 

 can be examined by night with the same perfection and accuracy as 

 in daylight, without that fatigue occasioned by the monochromatic 

 light ; moreover, colours are distinctly recognised. 



The modes of applying this contrivance are numerous ; a sub- 

 stance soluble in tallow, and supplying the requisite tint during com- 

 bustion, may be added in the formation of candles ; or another head 

 may be added to the illuminator of opake objects, and a plate of glass 

 of the proper colour fixed in it, this being placed as near the light as 

 possible, and between it and the mirror of the microscope. Again, 

 a piece of the coloured glass may be fixed in the condenser ordina- 

 rily used for transparent objects, or the lamp-glass itself may be made 

 of the requisitely coloured material. The method of effecting this 

 properly is to select the coloured glass by daylight, illuminating the 

 microscope by a candle or lamp, and then placing the glass between 

 the light and the mirror ; the field will, if the glass be of a proper 

 colour, appear of the same tint as the cloud light. 



[We are informed by our correspondent, that in the preserva- 

 tion of objects for the microscope in the liquid state, a solution of 

 Canada balsam in sulphuric aether, of such a state of viscidity as is 

 just sufficient to allow it to be laid on with a pen or stick, has an- 

 swered better than any of the means published in his former paper*. 

 Another very excellent mode is using gold- size and white lead ; this is 

 an excellent compound for the same purpose. — Ed. Ann, Nat. Hist.'] 



* Page 113 of the present volume. 



