by means of ordinary Artificial Liyht. 177 



the dulness of their colour by transmitted light, render them 

 unsuitable for this use. " Coloured" glass is iveQ from the 

 defect of refracting red. 



The paint ought to be applied so as to be as transparent 

 as possible, unless it be intended to be dim, in order to dif- 

 fuse the light. 



It is most conveniently applied with oil, which ought to be 

 as free from colour as possible, and to be made very dry- 

 ing. As it dries slowly, and ought therefore to be used 

 sparingly, mastic varnish or other varnish may be used along 

 with it to dilute the paint. But the oil of turpentine, or the 

 alcohol of the varnish, go off so quickly that the fluidity of 

 the paint varies very much, and it is difficult to apply it 

 smoothly and uniformly of the proper tint. Or, without using 

 varnish, gum mastic may be dissolved in the oil by heat, at 

 as high a temperature as can conveniently be used in paint- 

 ing the glass, perhaps 70° or 80°, or more, which makes much 

 less oil necessary than would be required at a lower tempe- 

 rature. By applying the paint in several coats, allowing each 

 to be thoroughly dry before applying another, instead of ap- 

 plying one thick coat, the precise tint required is more easily 

 produced, and the drying of the oil is promoted. If the paint, 

 when finished and dry, be deficient in transparency, a coat of 

 mastic varnish or other varnish may be applied, but cau- 

 tiously, so as not to dissolve and disturb the paint. 



Another method is, without using oil or varnish, to mix 

 the paint in the state of powder with gum mastic melted by 

 heat, which requires to be above the boiling point of water, 

 and to keep up the necessary temperature while painting. 

 By this method, the paint is fully as transparent as by the 

 other, it becomes hard immediately upon cooling, and it does 

 not refract red. 



In order to adjust the depth of tint of the glass, whether 

 " coloured" or painted, the white light of the sun may be re- 

 ceived on a piece of white paper, placed at a short distance 

 behind a window, which is to be used as the standard of white 

 light. The paper ought to be as purely white as possible, 

 and a blue tinge ought, in particular, to be avoided. The 

 light may be conveniently obtained by excluding the external 



VOL. XLH. NO. LXXXIIL— JANUARY 1847. M 



