March, 1903.] 



KNOWLEDGE 



69 



When everything has been prepared, and the Bjiecimen is in 

 place, immersed in plenty of the liquid jelly, the cover-glass is 

 taken up vfith the forceps, and gently lowered on to the jelly, 

 beginning from the left-hand side, driving the jelly (and too 

 often, alas, the specimen also) before it as it is allowed gradually 

 to fall into place. This is an operation of no little delicacy, as if 

 great care is not taken a large bubble of air will make its way 

 in at the last moment. If, as frequently happens, the putting 

 on of the cover-glass has caused a displacement of the object, 

 this must be rectified before the jelly is allowed to set, and here 

 the bent di.sseoting-needles will be of great service, as a consider- 

 able amount of re-arrangement can be eif ected with one of them, 

 and stray air-bubbles may also be removed without disturbing 

 the cover-glass. The slide is now taken from the bath and 

 allowed to cool, and in a few minutes the jelly will have so far 

 solidified that the mount can be examined under the microscope, 

 when, should any serious defect be disclosed, the jelly must be 

 re-melted, and the shortcoming be rectified. The fin:il process 

 consists in removing the superfluous jelly from around the 

 cover-glass with a knife, cleaning the slide from all trace of the 

 jelly, a handkerchief moistened at the li[is is the most efiicieut 

 method, and sealing the cover-glass round the margin with 

 some kind of varnish. I have tried a good many sealing 

 materials, and, on the whole, much prefer copal varnish, as 

 sold by artists' colourmen, thinned with a little benzine. One 

 special advantage arising from its use is that if any portion of 

 the object happens to be fixed near to the edge of the 

 cover-glass, it can nevertheless be seen through the practically 

 colourless varnish. I may add that I usually mount two cells 

 on each slide ; in the larger of them I place a small portion of 

 the moss, together with a few capsules, if possible in various 

 stages of growth, and two or three perichoetial leaves, while the 

 other cell contains some leaves dissected from the plant (where 

 of importance from both stem and branch), and a few pieces 

 cut from the mouth of the capsule to show the peristome. 

 (To be continued.) 



Monochromatic Light. —The use of light filters or screens 

 has of late become increasingly appreciated, and numerous 

 attem])ts have been made to secure a light filter of definite 

 value, which shall pass a maximum quantity of light. A fresh 

 interest has been added to the subject by the method of obtaining 

 monochromatic light which was exhibited by Dr. E. J. Spitta at 

 a recent meeting of the Royal Microscopical Society. 



It has to be remembered that strictly monochromatic light 

 — that is, a selected ray of the spectrum of one uniform wave 

 length — cannot be secured by light filters only. All the manu- 

 factured screens are only approximately monochromatic — that 

 is, by the judicious selection of ingredients they absorb certain 

 undesirable colours. 



Practical monochromatism can, however, be secured with a 

 suitable arrangement of prisms, and this, undoubtedly, is the 

 most precise method of working. It is not po.ssible. however, 

 with the majority of workers for prisms to be brought into 

 regular use, and a recapitulation of the best screens that are 

 known will be of interest. 



In passing, the advantages of light .screens or filters may be 

 indicated. In the first place the possibility of a lens to divide 

 fine detail is in a measure increased. Mr. Nelson has stated 

 that monochromatic blue light increises the effective working 

 of objectives of low aperture 14 per cent., but beyond 'Dlt N.A. 

 the value in this connection is not worth consideration, but it 

 is of special value in producing a clearer image of the specimen 

 under examination. 



In effect, if a light of shorter wave length than that possessed 

 by white light be used for illuminating purposes, whether 

 obtained by prisms or screens, the effective aperture of a lens 

 is increased. A reference to a numerical aperture table will 

 show this at once, the lines per inch resolvable by blue light 

 being considerably in excess of those resolvable by white light 

 with identically the same lens. 



(To be conthnied.) 



D1S.SHCTINQ Needles. — A dissecting needle of suitable shape 

 is often of great convenience. The following brirf note may be 

 of interest. If it is desired to bend it, it should ln' boated to a 

 dull red and allowed to cool gradually, it can then be bent into 

 any shape, and if desired an edge cjin bo ground or lilod. To 



re-harden, heat as before to blood-red heat, and plunge into 

 cold water. In this condition it will be found too hard and 

 liable to easily break. It should be rubbed bright on fine emery 

 paper, then held in a spirit lamp until it assumes a pale straw 

 colour, and again dipped into cold water. The correct temper 

 for use will then be obtained. 



Practical Sciiemi:. — A correspondent has sent from Oporto 

 for distribution a quantity of the protecting case of the flower 

 cone of Pinui pinaster. The scales can be either mounted in 

 balsam, or viewed as opaque objects. I shall be pleased to send 

 a small quantity to ap])licants sending a stamped addressed 

 envelope, accompanied by the coupon to be found in the 

 advertisement pages of this issue. 



NOTES AND QUERIES. 



G. Damant (Cltinri). — On examining the specimens in the tube 

 which you have sent, I found that two parasites were adherent 

 to the abdomen of the cockroach. These are larvae of the genus 

 Rhipidoi)horid8e, and the winged specimen is the perfect insect. 

 These parasites are nearly allied to the British Mftoecufi 

 parailiKi-ax, which is a parasite of the wasp. You cannot do 

 better than proceed with the mounting of your insect parts in 

 the manner described by Mr. Cole in Modern Microscopy. In 

 order to overcome your difficulty with the insect mouth organs, 

 etc., you will proceed as far as the clove oil stage, then take a 

 slide and place the parts upon it in the desired position in a very 

 little clove oil ; generally there will be enough oil about each 

 part without adding any more. Now place a cover-glass over 

 all, being careful that the specimens do not move, then ap])ly 

 some thin Canada balsam solution to the edge of the cover and 

 let it run under so that it goes all round the soecimens. There 

 will probably be some air-bubbles, but they should be dis- 

 regarded, they will disappear as the balsam dries. Put the slide 

 aside for, say, fifteen minutes, then add more balsam, and so on 

 until the space between the cover and slip remains entirely filled 

 up with balsam. It should then be allowed to dry for a few 

 days, wash oft" any excess of balsam with benzole, apply a coat 

 of black varnish, let this dry, clean the slide with turpentine, 

 and add another coat of varnish. 



G. V-G. — All the objectives you name are high-class, but I 

 am inclined to recommend for your special purpose the one 

 made by Leitz. The lenses of smaller aperture for work on 

 Diatoms would certainly not be so advantageous. 



W. D. D. — The parasite of the Hydra fusca is T'richodina 

 pediculus. 



T. P. Thomson. — I am much obliged for your proposal, but 

 am afraid that Diatoms of your locality would not be of 

 general interest unless indeed you had discovered some new or 

 very rare species. 



G. F. Broivn. — If you will kindly send me another sample of 

 the Polyzoa, I will endeavour to identify it for you. 



ir. F. Albroiv. — I have communicated with the consultant 

 who interests himself in the particular department to which 

 your query refers, and he suggests that the peculiar substance 

 which you have found in your flagon of distilled water may be 

 due, as you suggest, to a dirty vessel, in which case the growth 

 would perhaps be a fungoid or gelatinous bacterial one, de- 

 tached from the bottom of the vessel by the act of emptying it. 

 He further suggests that it might perchance be due to the 

 solvent action of the water detaching portions of the surface of 

 the glass. This is common with alkaline solutions, especially 

 ammonia, though the large size and circular shape of the 

 described growth are difficult to reconcile with this idea. If any 

 of the substance is still obtainable, the consultant would be 

 ]iarticularly interested in examining it. 



Communications and enquiries on Microscopical matters are 

 cordially invited, and should be addressed to M. I. CROSS, 

 Knowledge Office, 326, High Holbom, W.C. 



NOTES ON COMETS AND METEORS. 



By W. F. Dennino, f.b.a.s. 



Q-iACOBiNi's Comet (1902 n).— Thia object continues well risible 

 in |iretty good telescopes, though it is decidedly small and incoo- 

 spicuous. In March and April the comet will travel slowly to the 



