H. Onslow 213 



unit of onuiffi" ; and a yolli>w-unin^t' colour may ha uuuiv by ct)iiibiiiing 

 one unit of nil with twt) or more uniU* of yt'llow. Further, one unit of 

 nnl + one unit of yellow + »)IK' unit of bhu* = one unit of neutral tint, or 

 black. 



It is affiruKil by tho nuikers that the thn'r c«)|our-uiiits whi<!h ^o U> 

 make up the neutral tint are e<|ual, that is Ut nny, when time ditVrrent 

 eolourtnl units an* conibiniHl then* is n<» residual colour. All units and 

 fractions ai*e siiid to be cheeked by this test., and, further, all the glasses 

 of one set are interchanged with those of another and the units verified 

 by CHKss checking. To enable obsi'rvers to verify their scales the makers 

 publish colour readings, obUiined by dissolving a known weight of a 

 pure substance such jus |M)tjissium-ferricyanide in a giv«'n voIuiih* of 

 water, a known depth of which can bi' examined in cells of ditt'erent 

 thicknessi'S. The only objection to this procedure is that even s(j|utions 

 of the most stable substances such jus picric acid are com|j{iratively 

 inconstant, and may show either a fading, or an increiuse of colour on 

 standing. 



In tuxler to obtain the colour measurement of a given in.sect it 

 should be pinned upon a strip ot cork and su placed that the desire(J 

 portion of the wing, magnified by the lens, comes immediately under 

 one aperture. A suiUible diaphragm is then chosen to cut out most of 

 the black markings, while leaving exposed the central yellow portion of 

 one wing. A red glass slip of a certain value is next chosen, and this 

 is combined with a yellow slip, representing about twice as many units. 

 Such a combination will give a bright orange which may be either too 

 intense or too dilute. By the selection of suitable units or fractions 

 both the red and the yellow are alternately increased and decreased, 

 till a colour exactly matching the wing is obtained. Care should be 

 taken to limit the length of each observation to five seconds, in order to 

 avoid the disturbing effects due to fatigue. It is often found impossible 

 to obtain a perfect match with the red and yellow colours only, owing 

 to the fact that the colour of the wing is desjiturated by the addition of 

 black. In this event the exact colour e(jui valence may be obtained by 

 adding some fraction of a blue unit, e.g. when the units on the slips 

 of each colour employed in obtaining a {x^rfect match are added together, 

 the toU\l may be found to come to : 



Red Yellow Blue 



ry6 : 9-8 : 0*4 



These colours will not however be the same as those appreciated by 

 the eye. When converting the former into the latter, as must always 



