8 



KNOWLEDGE. 



[January 1, 1897. 



method on pieces of cardboard, are liable to serious 

 error unless the greatest care be taken to prevent the 



Fig. 6. 



Fig. 5. — An insect witli its wings on the riglit hand properly set, 

 those on the left showing contraction due to careless setting. 



■wings from being drawn up towards the thorax as the gum 

 dries, the consequence being considerable shortening and 

 distortion of one or both wings. 



Fig. 5 shows an insect with its wings on the right-hand 



side properly expanded 

 and " set," the tips just 

 touching the card. The 

 left-hand wings have 

 been drawn in by the 

 contraction of the gum, 

 thus not only fore- 

 shortening the wing, 

 but preventing the short 

 costal nerve from being 

 seen. This is the cause 

 of many mistakes. 



Figs. G and 7 repre- 

 sent two minute Hy- 

 menoptera, upon whose 

 structure so little detail 

 has been shown that most strange errors have crept in 

 concerning their identity, and handed down and copied by 

 authors, who, by so doing, prove howvery little attention they 



give to studying 

 these creatures, 

 except from books. 

 In conclusion, 

 I would urge upon 

 all entomologists 

 and others the 

 vast importance 

 of acquiring and 

 pursuing the habit 

 of making most 

 careful observa- 

 tions anddra wings 

 of these small in- 

 sects, about which 

 we Icnow so Uttle 

 when we ought to 

 know so much. They are our best insect friends, without 

 whose help, by keeping in check noxious pests, we should 

 scarcely be able to exist. 



In future articles I propose to describe some of our 

 common insects whose life-histories have not yet appeared 

 in print. 



Fig. 7. 



THE SPECTRUM TOP. 



By T. L. Alger, LL.D., Ph.D. 



AS I look around me, and into the sayings and 

 doings of workers in the fundamental principles 

 of Ught, I do not find that a perfect synthesis of 

 white light has been obtained so as to embrace 

 the law of the combination of pigments. The 

 principle of selective absorption does not appear to me to 

 entirely settle the matter. 



It is patent that the crossing of the elementary beams 

 of the solar spectrum — the red, the green, and the violet 

 rays — can be made to coalesce on a screen and form a 

 patch of Ught which has more or less a bluish or greenish 

 fringe ; but that, when the same colours as pigments, or 

 their nearest approach, are marked out on a disc, and set 

 revolving by means of a multiplying wheel, the effect pro- 

 duced is not pure white, but the grey of a combination 

 similar to that of a disc divided according to the same law 

 in black and white, and set revolving in a similar manner. 



Now, the question occurred to me, was it possible to 

 place a tricolour painted disc under circumstances that 

 could produce a pure white light ? 



I first took a disc of white cardboard about two inches 

 in diameter, and divided it into two equal portions by a 

 line passing through the centre of the disc, and painted 

 one-half of the disc black. I then divided a similar disc 

 into four quadrants, and blackened the alternate ones, and 

 another similar disc into eight half-quadrants. On ap- 

 plying the multiplying wheel in all these cases, I produced 

 the same tint of grey as would be produced by mixmg in 

 equal proportions black and white pigments. I therefore 

 came to the conclusion that the equal distribution of black 

 and white on the disc, under different arrangements, pro- 

 duced the same effect — the grey. In these experiments I 

 failed to trace the slightest prismatic colour. 



On covering other and larger discs with red and blue, 

 red and yellow, and other combinations of what in Newton's 

 days were known as primary colours, I found a similar 

 effect, /.('., the red and blue produced violet, the red and 

 yellow, orange, and the blue and yellow, green : (.<?., the 

 same tint was produced in each case as would have been 

 produced by mixing the pigments. It was therefore clear 

 that rapidity of revolution in layers of equal or unequal 

 intensity produce the same effect as their combination 

 when mixed together as pigments. 



How, then, are we to reproduce the resultant beam of 

 white light without a fringe by uniting the several com- 

 ponents, taking them optically as red, green, and violet ? 

 I now divided a disc of about four inches diameter into 

 three equal portions, and painted red in one, green in the 

 next, and violet in the remaining one, taking care to get 

 the tints as near as possible to those of the solar spectrum. 

 On rapidly twirling this disc the same effect was produced. 

 I had the grey that would be formed by the mixture of 

 the three pigments. I was therefore bound to come to the 

 conclusion that experiments, if they can be manipulated 

 so as to produce white in pigments, must be performed 

 with the aid of prisms. 



I now took another disc, and divided it into six equal 

 portions by lines passing through the centre, and sub- 

 divided one of the sectors also into six equal portions so 

 as to take two opposite sectors of seven-sixths for the red, 

 two opposite sectors of five-sixths for the green, and two 

 opposite sectors of six-sixths for the violet, making up the 

 six sectors. This is Newton's law, but Newton used what 

 were then considered to be primary colours — red, yellow, 

 and blue. 



