178 G. Tait, Esq., on producing White or Neutral Light, 



light from the apartment, leaving an opening of a proper 

 size near the lower part of the window, and covering the 

 opening with purely white tissue paper, which receives the 

 external light, and transmits the greater part of it, so that a 

 portion of it is received upon the standard paper. The light 

 received upon the tissue paper may be either the direct rays 

 of the sun, or diffused light from white or neutral grey clouds 

 or less condensed vapour, or during rain or fog, but not from 

 blue sky. In either case, the observation ought not to be 

 made when the sun is low, on account of the warm tinge 

 which the light usually has at that time, or when there is a 

 tinge from smoke. The tissue paper may be dispensed with, 

 and diffused light may be received directly from without upon 

 the standard paper ; but the method before described is, in 

 general, more convenient and satisfactory. 



Another piece of similar paper is then to be exposed to the 

 artificial light transmitted through the glass, being, at the 

 same time, completely shaded from the day light ; and, if 

 " coloured'' glass be used, it is to be procured or made of 

 such a depth of blue that the latter paper may appear as 

 white as the standard ; or, if painted glass be used, it is to 

 be brought to produce the same effect, making allowance for 

 the temporary green tinge before referred to. The more 

 correctly that this paper is made to agree with the standard, 

 the better. A greater degree of blueness is, in particular, 

 to be avoided. The two papers are more easily compared if 

 they be made both to receive the same quantity of light. 



If, from the flame and the paint used not correctly neutral- 

 izing each other, or from any other cause, there be a little of 

 a permanent green tinge, it may be neutralized by the addi- 

 tion of a little transparent red ; or, if purple, by transparent 

 yellow. The same principle applies to the adjustment of the 

 proper tint of " coloured" glass. 



If the light pass obliquely through a glass, or part of a 

 glass, less colour is, in consequence, necessary. 



After the paint is completely dry, the light transmitted 

 ought to be again compared with white light, and any little 

 inaccuracy corrected. 



When one glass has been adjusted, others, whether 



