214 



KNOWLEDGE. 



[September, 1901. 



micrography but are appalled by the number of fittings that are 

 usually specified as necessary. It has to be borne in mind that 

 it is not everyone who is bent on performing feats of resolution 

 of diatoms or working with high powers ; in making observa- 

 tions there are numerous enthusiastic amateurs, who, seeing a 

 striking feature in an object, wish to record the appearance by 

 ])hotographing it for future reference or for the edification of 

 friends. The bulk of such work would lie within the scope of 1" 

 or -J' objectives, and many of the ordinary fittings of complete 

 ]ihoto-micrographic cameras are not really necessary, however 

 desirable they may be. 



What is the simplest apparatus that can be used ? 



Being provided with a microscope and bull's eye, the addi- 

 tional apparatus would consist of an oil lamp, some forrn of 

 camera, a support that will carry the camera at the axis height 

 of the microscope when set horizontally, and a tube to connect 

 the front of the camera with the body of the microscope. 



1. The Lamp. — This need not be of an expensive kind such 

 as is ordinarily supplied for microscopical work. One with a 

 tin frame which is purchasable at an oil shop for Is. answers 

 very well. 2. The Camfni.— This should not have less than 

 10" of extension and requires to have a good dark slide with it. 

 The focussing would, of course, be done by means of the 

 microscope adjustments. 3. The connecting tube might be of 

 cardboard or of brass as preferred. 4. A support for the Camera. 

 — This is really of great importance in an arrangement of this 

 description. It would be well that a frame should be shaped 

 up by a carpenter to which the camera could be attached and 

 on which, at a suitable level, the microscope and lamp could be 

 placed in a fixed central position. 



A knowledge of ])hotography is so general that it is hardly 

 necessary to say that an acquaintance with develojiing, printing, 

 etc., is desirable ; but with such contrivances as those named, 

 carefully arranged, really good results can be secured with low- 

 power objectives. 



Success under such circumstances will lead to a desire to 

 attempt more pretentious work with better apparatus. Photo- 

 micrography has a charm of its own, but in order to attain 

 proficiency it is essential that the worker possess a good practical 

 acquaintance with microscopical manipulation and ordinary 

 photography. The failures that are so frequently met with are 

 due in the majority of instances to ignorance of one or both 

 subjects, in fact, the writer has met with people who have 

 essayed to do photo-micrography without previous serious work 

 with the microscope or knowledge of its limitations. A serious 

 practical man can succeed with elementary arrangements where 

 a novice with the best equipment of apparatus would fail. 



Low-priced Objectivks. — Perhaps in nothing so much as 

 in scientific work is the fear present that anj'thing that is low 

 in price is likely to be poor in quality and yield corresponding 

 results. But it can be claimed that this does not apply to 

 microscopical objectives. 



The past decade has seen a wonderful forward movement in 

 the construction of these lenses. The almost general use of 

 the microscope in laboratories, technical schools, for manu- 

 facturing purposes, and even in primary schools, added to the 

 increasing recognition of its value as a means of recreation, has 

 created a demand which has been at all times a critical one. 

 There have always been the few who have been quick to recog- 

 nise merit in lenses and to make their virtues known. Manu- 

 facturers have vied with each other, and by the use of the 

 many optical glasses that are at their disposal, combined with 

 extensive computations and experiments and skilful work, they 

 have produced lenses of large aperture and fine performance. 

 When apochromatic objectives were first introduced their per- 

 formance was in the majority of cases immeasurably superior 

 to others. These have been taken as a standard, and many 

 lenses at very low cost are being made that are practically equal 

 to them excepting only so far as the colour corrections are 

 concerned. 



It must not be inferred that this point is neglected, in fact 

 it requires a critical eye oftentimes to detect an inferiority in 

 this respect, but for the bulk of the work undertaken by micro- 

 scopists it can be disregarded, and the cheaper lenses will reveal 

 all that the apochromatics are capable of showing. These 

 remaiks apply to the products of nearly every firm of repute, 

 and advice would be willingly given by such in the selection of 

 lenses, which could be accepted without misgiving. 



NOTES AND QUERIES. 



.r. E. S. — The reason why a condenser having a smaller aper- 

 ture than that possessed by the objective is sufficient is because 

 there are no lenses available that will bear an illuminating cone 

 equal to their own aperture. As a rule definition begins to fail 

 even with the best lenses when anything more than a cone 

 of three-fourths the numerical aperture of the objective is em- 

 ployed. There have been exceptional lenses which have borne 

 more than this, but they are rare. It would be well worth your 

 while to have an oil immersion condenser if you were using an 

 objective with a X.A. of r40. It is possible that this lens 

 would stand a cone slightly in excess of I'O N.A., and the best 

 dry condensers do not give much more than '9 The cone of 

 illumination required depends in a large measure on the sub- 

 jects that are under examination ; for a large amount of work 

 a cone of 'it would be quite sufficient for a lens of r4il. 

 Directions for the regressive staining method are contained 

 I fully in the " Jlicrotomist's Vade Mecaiii" by Lee. Several 

 i pages are there devoted to the subject. The process could not 

 be briefly described in these columns. 



A. Glaister. — I am sorry to say that when the tubes containing 

 the Infusoria you so kindly sent reached me, the contents were 



1 dead and quite disintegrated. It was therefore impossible for 

 me to identifj' any of the specimens. Should you wish to send 

 others, perhaps you would let me have advice a few days in 

 advance so that I could arrange to receive and examine them 

 promptly. 



B. IL J. (-I///.<ior« Province). — When it is desired to show the 

 contents of cells and vessels in wood.s, the sections must be cut, 

 and mounted, dry. The best tools for making large sections of 

 hard wood, are a carpenter's bench and a good plane. Longi- 

 tudinal sections of almost any size can be cut in this way, and 

 the}- can be trimmed up and the ends cut square with a pair of 

 scissors. Then, if desired, they may be soaked in water and 

 mounted in glycerine jelly. If the cell contents, viz., oils, 

 turpentine, or resin are required to be shown, the sections must 

 not be placed in alcohol because it would dissolve such contents : 

 they consequently cannot be mounted in Canada balsam. I do 

 not know of any microtome that will cut transverse sections of 

 hard tissues of a large size. The one you refer to — Cole's 

 pattern — is a very good one, but I am afraid that paraffin would 

 not do for embedding, especially in India : a good firm piece of 

 carrot is the only thing I know of. For instructions in staining 

 and mounting, I would recommend Lee's " Microtomist's ]'ade 

 Mecum," and for practical botanical histolgy Strasburger's 

 "Practical Botany'' (Swan Sonnenschein & Co.). I do not 

 know of a book on entomology suitable for microscopists. 

 The makers of lathes, etc., for grinding and cutting sections 

 are Messrs. Cotton & Johnson, of 14, Gerrard Street, Soho, W., 

 whom I have asked to send you a price list. A good practical 

 work on the subject is " The Study of Rocks," by Butley 

 (Longmans). 



Communications and enquiries on Microscopical matters are 

 cordially invited, and shotdd be addressed to M. I. Cross, 

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



NOTES ON COMETS AND METEORS. 



By W. F. Denning, f.r.a.s. 



Encke's Pkeiodical Comet. — The re-detection of this comet is. 

 announced by Prof. Wilson, who saw it on August 6tli, in the 

 aiiproximate position R.A. 65", Dee. 31° 43' N., so that the comet 

 was close to (i AndroraediE. The period of revolution being 3'3 years, 

 the returns of this object occur at intervals of 33 years (10 periods), 

 when the eu'cumstances of its visibility are nearly the same as before. 



Comet 1897, I- (Peeeixe).— Dr. Johannes Miiller has investigated 

 the observations of tliis comet extending over the period from 1S96, 

 November 2. to 1897, April 29, and deduces hvperbolic elements. He 

 fiuds the exceuti-icity l'U00927, and gives a table in wliich is compared 

 tlie orbital elements of seven other hyperbolic comets, viz. : 1844, III., 

 1886, IL, 1889, I., 1890, II., 1892, II., 1898, VIL, and 1899, I. 



Eenst August L-IMP. — Dr. Lamp, of Kiel, whose name is well 

 known in couuection with coiuetary investigations, died on May lOtli, 

 1901, aged ,51 years. Included in his valuable labours were a com- 

 putation of the definitive elements of Comet 1891, I , and a discussion 

 of the supposed connection, first pointed out by Mr. Hind, between 

 Brorseu's Comet (1846) and Comet 1894, I. (Denning). He gave his 

 conclusions ou the latter subject in Ast. Nach , 3278, and showed that 



