606 



• KNOWLEDGE • 



[May 19, 1882 



the rpquirod position, or sufficiently near it ; if not, both would 

 require to bo moved a little so as to xatisfy the above condition. 

 When that was obtained, all that remained was to take the distance 

 between E and P by a pair of compasses, and determine its value 

 from a soalo. I have an excellent ivory scale, divided into fortieths 

 of an inch ; and to save trouble, I assumed this quantity as the 

 unit of my moasuroments. Havinf; made the simple and rude 

 arranfjomcnt described above, I attempted the oi)eration, E and F 

 boinij stuck with their points into the cork discs. My linst trial, 

 however, proved that under a deep lens their blunt ends appeared 

 too coarse to admit of accuracy. I therefore stuck them the other 

 way into the corks, with the points upwards, which answered much 

 better. My first trial, with the blunt ends, gave the distance 

 from E to F 30 parts of the scale ; the second, with points, 

 31 ; both by candlelight. The next morning five trials gave 

 31106, 31-GOG, 310, 30066, 310. (When the distance taken by 

 the compasses did not e.\actly correspond to any division of the 

 scale, 1 repeated it on a paper by the edge of the scale till a coin- 

 cidence was obtained, upon the principle of the vernier.) The lens 

 was plano-convoi, and these trials had been made with the convex 

 aide towards the eye ; it struck me that, on account of the aberra- 

 tion, it ought to be turned the other way, which was done, though, 

 in fact, it was needless, as I was mistakenly reasoning from the 

 case of parallel'rays. I then commenced a fresh set of trials, which 

 gave me the following results :— 320, 31-333, 31-5, 31-5, 31-25, 31-8, 

 31-5, 31-3, 310, 31-0, 31-0. It should bo mentioned that, after every 

 trial, the discs carrying the needles were moved from their position, 

 and the succeeding trial was commenced entirely afresh. It was 

 soon apparent that the method deserved confidence, and the results 

 agreed more nearly as my eye and hand became accustomed to the 

 operation, the last three exactly coinciding ; whence I preferred 

 their value, as an ."approximation to the truth, to a mean of the 



whole, and deduced ——=7-9 (fortieths of an inch) as the focal 



length of the lens for parallel rays. 



I will now mention the difficulties and the advantages of this 

 method. The greatest difficulty is in measuring the distance 

 between the points of E and F with compasses. Thi.s, perhaps, 

 could only be done by a very near-sighted person like myself (or by 

 the aid of suitable spectacles), and required practice and steadiness 

 of hand. Another source of error lay in the possibility that the 

 points E and P might not lie in the axis of the lens, but might be 

 originally fixed too high or too low, or might slip on one side, as, 

 indeed, frequently happened from the turning of the cork discs 

 upon the knitting-needle as an axis. Another difficulty consisted in 

 estimating the position where E and F should be equidistant from 

 D. The situation of the latter in (the depth of) its cell could not 

 be very plainly seen, and as its thickness was considerable, it was 

 uncertain from what point in it they should be equidistant. 



It must be observed, however, that every one of these sources of 

 error is to be ascribed, not to the method, but solely to the defects 

 of 80 rude and imperfect an apparatus, which might easily and com- 

 pletely be remedied. A microscopic eye and steady hand were the 

 only means I employed to obviate them, and, as the results show, 

 not without success. As I became more familiar with the opera- 

 tion, I guarded more carefully against the second source of error; 

 and hence, probably, arose the increasing coincidence of the later 

 determinations ; the third difficulty, which I found least remediable, 

 was not likely to produce any injurious effects. For calling x the 

 principal focus between E and D, and y the principal focus between 

 D and P, we have, by Cor. 3 t» the before-cited Proposition,— 

 Ex : j)D : : Di/ : yP. Supposing now that E has been placed at a 



distance of 14-8, instead of 15-8, from D, we have vF=- — -"y = 



7 9i' " -^^ 



^g =9 nearly, and EP mil be = (6-9-! 7-9 -h 79 -H 9 = ) 31-7,instead 



of 31-6, an inappreciable difference. Or if ED was only 14 (and a 

 greater error is very unlikely, since that would make DF = 18-1), 

 still EF would be only 321, which would make but about 012 of 

 an inch difference in the focal length. 



The advantages of the method are— 1. The accuracv with which 

 the place of the conjugate focus may be determined, "if the ima"-e 

 ^l '8 '"■""ght so that its point may be directly over, and close t°o, 

 that of F, an eye accustomed to telescopic observation will deter- 

 mmo, without much trouble, the situation in which both are most 

 distinct at the same time ; and this might be reduced to "rcater 

 certainty, if the lens with which they are examined is fixed in a 

 slide instead of being hold in the hand. 2. The elimination of the 

 thickness of the lens whoso focus is to be measured— a very trouble- 

 some quantity, which it is a great advantage to get rid of. 3 That 

 the errors of observation are diminished to one-fourth in the final 

 result ; the measured distance being four times the focal length 

 required. ° 



BUTTERFLIES AND MOTHS. 



THE science of Entomology, during the last few years, has made 

 enormous progress, both in regard to the number of its de- 

 votees, and also in the discoveries of the best methods of obtainipg 

 and preserving the insects. We purpose, therefore, giving a few 

 notes weekly relating principally to the order Lopidoptera, aboat 

 the haunts, times for catching, food, &c., of our British butterflies 

 and moths, which one may generally expect to find during the 

 course of the year. 



The present may be called the opening month of the season, and 

 the entomologist will find plenty of work to do, both day and night, 

 in the lanes and wood.s of the country. A bright May-day will most 

 certainly bring numbers of butterflies to the not of the energetic 

 collector who keeps his eyes open, and who can manage a sharp 

 walk over fields and lanes without feeling the fatigue. 



The first thing for the would-be entomologist to do is to obtain a 

 net, a few setting-boards, some entomological pins, and two or 

 three dozen chip-boxes, and then he can consider himself well set 

 up to begin with. All these articles he can make himself, with the 

 exception of the pins and chip-boxes, which can bo obtained at a 

 small cost of the dealers in natural history implements and speci- 

 mens. The setting-boards mentioned are pieces of soft pine aboat 

 fourteen inches long, and of various widths, with a cork face, and a 

 groove cut down the centre.* Entomological pins are a very fine 

 and .sharp-pointed kind, made specially for this purpose ; they can 

 be obtained in assorted sizes at Is. per ounce, one ounce lasting 

 most people, an entire season. 



We will now suppose the insects to have been canght, and the 

 momentous question of how to kill them crops np. For butterflies, 

 the readiest method is to use a killing bottle composed of cyanide 

 of potassium, covered over with a layer of plaster-of-paris in a wide- 

 necked bottle ; the insect is killed very quickly, but unfortunately 

 the wings get set equally rapidly. Another way is to squeeze the 

 thorax of the victim whilst in the net. This kills immediately, but 

 has the disadvantage of slightly damaging the insect. For moths, 

 the best plan is to stupefy with chloroform, and then prick them 

 just below the thorax, or the under side, with a sharp pen dipped in 

 a saturated solution of oxalic acid. 



Setting is the uw ' f>rocess. It is accomplished in the following 

 manner: One of the entomological pins is thrust through the moth 

 near the head and the wings stretched out,* and then a narrow 

 strip of cardboard pinned over to keep them in position. This 

 operation requires great care and neatness, and must be done very 

 shortly after the death of the insect, or the wings get set, and can- 

 not be moved without considerable trouble and delay, and very often 

 damage to the moths during the process of relaxation. This latter 

 will be described in a future paper. 



The insects must be left on the setting-boards for a period 

 varying from four or five days upwards, according to the size of 

 the moth or butterfly and the length of time the boards can be 

 spared. Personally, whatever the size of the moth, we let it 

 remain until we require the board again, which is generally at 

 least a month or six weeks after, as we keep a large number f 

 boards in use. 



The different stages of the work required to be done in killing 

 and setting the insects having been described, we will pass on to 

 the manner of keeping the specimens in good order and condition. 



Our first advice to the tyro is, not to get a cabinet unless he can 

 afford to buy a thoroughly good one, with air-tight and dust-tight 

 drawers. 



By far the best plan is to use store boxes of moderate size, 

 either plain or covered with green baize, as they aie perfectly air- 

 tight, and do not allow the light to penetrate, which is a very great 

 consideration, owing to the colours of some moths being very un- 

 stable, greens and other light colours often becoming white in a 

 very short time, owing to exposure to the action of light. However, 

 if getting a cabinet is decided on, we repeat, let it be a good one, 

 icith n-ell-ftting draivcrs. 



The arrangement of specimens depends to a great extent on the 

 taste and fancy of the collector; but the most scientific and satis- 

 factory method is to follow the order of genera, families, and sub- 

 families, which the best naturalists have adopted. Xext week we 

 will give this classification in full, as space forbids in this paper. 

 A piece of camphor about the size of a walnut must be put in each 

 box or drawer to keep off the mites, whose ravages have sometimes, 

 in one single winter, destroyed or damaged the w-hole work of the 

 preceding summer, when this precaution has been neglected. 



Everything now is done connected with the indoor work of the 

 entomologist, and next week we purpose describing the different 

 processes used for catching both diurnal and nocturnal Lepidoptera, ' 

 together with their habits and other useful information. 



* Diagrams illustrating the above will be given in our next 

 number. 



