♦ KNOWLEDGE 



into our little mirror, C, we sliall see tlie field lighted 

 with the most lovely colours. We have said colours, 

 because the chances are considerably against the beginner 

 splitting off a film of identical thickness in all its parts 

 from the selenite. Should he succeed in obtaining one 

 of the same thickness throughout, then will the colouring 

 of the field of view be homogeneous. In the first case, 

 he will get patches of the most vivid and gorgeous yellow, 

 blue, red, and green, &c., — in the latter one of such 

 colours alone. Now let us turn our analyser slowly round 

 through 4-5°, and we shall find the colours, or colour, 

 gradually fade until the light will pass through the film 

 seemingly unaltered. Proceeding now to rotate the 

 analyser through another 45°, the colours will begin to 

 reappear, hi'.t they will be comi)lementartj to the original 

 ones ; that is to say, if the plate transmitted green 

 light in the first position of the analyser, it will now let 

 red light through. If it appeared blue originally, it will 

 now seem yellow, and so on ; the colours thus exhibited 

 always being those which, mixed together, form ordinary 

 white light. As the rotation is continued, the changes 

 described will recur (of course, in reversed order) until 

 the analyser has been turned 360° round the ray of light 

 as an axis. Mica, if suflBciently thin, will exhibit similar 

 phenomena, as will thin slices of quill or horn, tartaric 

 acid crystallised on a plate of glass, the frost ferns on the 

 window-panes in winter, and numerous other substances. 

 Let us, iu conclusion, see whether we cannot obtain some 

 general idea of the manner in which these most striking 

 and beautiful appearances are produced. 



If we remove the analyser, and look at the beam of 

 light proceeding from the polarizer through our film of 

 selenite or other material alone, we shall see such light ab- 

 solutely uncoloured. Hence it is obvious that the analysis 

 of this whole light, or its separation into colours, is 

 effected by the analyser. Now, selenite is a doubly re- 

 fracting ciystal, and when the polarised ray enters it, it 

 is resolved into two rays, vibrating at right angles to 

 each other. These rays are differently bent ; and bending 

 in a refracting medium means going more slowly, the ray 

 which is the more bent being the tardier of the two. 

 From this it will be readily seen that one of these rays may 

 get half a wave-length, or any odd number of half- wave 

 lengths, ahead of the other (Vol. VII., p. 321), and that, 

 as formerly explained (loc. cit.'), they may inf erf ere. A 

 little further consideration, though, will .show that while 

 these rays are composed of vibrations occurring at right 

 angles to each other, it is physicallj- impossible that 

 interference can occur. But the analyser twists the two 

 jilanes of vibration into coincidence, so that the waves 

 can now destroy each other. Selenite, or any other doubly- 

 refracting material, has, of course, a sensible thickness, and 

 we have seen formerly (Vol. VII., p. 541) that waves of 

 red light are much longer than those of blue. Conse- 

 f[uently we shall require a thicker plate of selenite to 

 retard the red rays sufficiently to extinguish them than 

 we shall if we want to blot out the blue rays. Hence, 

 when these longer waves have been neutralised by inter- 

 ference, the plate of selenite will shine with the colours 

 produced by the shorter ones. Vice-versd, when the 

 shorter waves have been annihilated, the longer waves 

 will get through the analyser, and the colour of the 

 field be derived from the less refrangible end of the 

 spectrum. 



Into the exquisite phenomena of circular and elliptical 

 polarisation, it is impossible to enter here. An account 

 of these must be sought in works specially devoted to 

 the subject. All we have essayed to do is to introduce 

 the student to the practice of some remarkable experi- 



ments which may be performed at little or no cost ; and 

 which serve to admit us to a nearer view of the intimate 

 nature of Light. 



OUR HOUSEHOLD INSECTS. 



By E. a. Butler. 



HYMENOPTERA. 

 (^Continued from page 264). 



THE ants referred to in our last paper are genuine 

 household insects, spending the whole of their lives 

 in the shelter of our abodes, breeding amongst us, and 

 bringing up their extensive families year after year in 

 the same spot, as long as provisions are plentiful in the 

 immediate neighbourhood. But this is not the case with 

 the wasps, the next section of the Hymenoptera which 

 will engage our attention. It is true that occasionally 

 their nests are found in outhouses or lofts, or under the 

 eaves of thatched roofs ; but this is exceptional, and, as a 

 rule, they enter our houses only in their adult condition; 

 still, they are then such tiresome pests — at least, in 

 imagination, if not always in reality — that we cannot 

 forbear to grant them a place amongst our household 

 insects. 



Notwithstanding the popular prejudice against wasps, 

 there are many points of interest in connection with 

 them. Their economy is remarkable, and inferior in 

 interest only to that of bees and ants ; their courage is 

 certainly extraordinary ; and though they are frequently 

 an annoyance to us through their intrusive habits, yet 

 there are, as we shall presently see, some counterbalancing 

 advantages following from their mode of life ; and, finally, 

 their character is not really quite so black as it has been 

 painted. That tlicy aru ur.t, as some people seem to 

 suppose, actu:;ii>l |.\ :,!, i;-,-ri.i .11 ilil>' I ,' - ' 1 1 1 1 y to human 

 kind has bun, :^ ■ . . . : ■ ■ M i - rvers who, 

 like Sir Join, I, ' I ., ^ ' . i : l-ii- habits, 



and have fuuuJ u j.>;.v.:,iL;u lu i.iuiu iIilui and make pets 

 of them, and to induce even such fiury-tempcrcd beings 

 calmly to feed out of their hands and to crawl over their 

 persons without bringing their murderous weapons into 

 requisition. Indeed, one observer. Dr. Ormerod, expressed 

 his opinion that they are much less fickle and more reliable 

 than bees — an opinion, however, which will probably not 

 be generally endorsed. 



They will rarely attack unless provoked, and, though 

 it is not easy to maintain a philosophic comjiosure and 

 indifference when a wasp is buzzing roimd one's head, yet 

 such would no doubt be the best policy ; at any rate, the 

 violent flourishes and dashes so often made against them 

 with handkerchiefs, knives, or what not are more likely 

 to irritate than to drive away insects so renowned for 

 valour. Of course, when we attack their citadel, they 

 will at once assume the offensive (as who would not ?), 

 and fight to the death for house and home. Very hot or 

 windy weather, too, seems to bring out whatever spite- 

 fulness they possess, hut this also is a psychological 

 experience not altogether foreign even to Eomo sajyiens 

 himself ! 



In distinguishing wasi^s from other Hymenoptera no 

 reliance must be placed on the mere presence of yellow 

 bands on the bodj-, for though all wasjis, of whatever 

 habits, have these, such a style of ornamentation is by 

 no means confined to them, but is of frequent occurrence 

 throughout the whole order. But there is a certain pecu- 

 liarity of the wings that will at once separate a wasp 



