March, 1902.] 



KNOWLEDGE 



69 



objective has i|ualities under so many jihascs, each Fcries of 

 lenses having tlieirown peculiar characteristics, that it is almost 

 impossible to have a standard test for all the iiualities, still it 

 would be a most advantageous and desirable consummation if 

 some centre could be arranged where undeniable unjirejudiccd 

 reports conUl be obtained conccrniug objectives. It might be 

 that an objective that was admirably corrected for spherical 

 aberration, well centred and |)osscssing a proper ratio of 

 aperture to m.agnification would be high in colour when the colour 

 tests were applied : this would not necessarily be a bad or inferior 

 lens, and might be superior to an apochromatic in which the 

 colour correction was -perfect, while the spherical aberration was 

 not so well controlled. Tests that are applied to objectives 

 should be as far as possible mechanical ones, that is, such as 

 will admit of but slight error of judgment, but this is only 

 partially possible. For instance, the numerical a])erture and 

 initial magnifying povrer can be determined, the former by 

 means of the Apertometer, and the latter by the regular rules 

 laid down in the text-books. Yet in so simple a ])roceeding as 

 measuring the aperture, there will frequently be a considerable 

 variation between the re?ults obtained by se|)arate individuals: 

 even this requires not merely care and skill, but a large amount 

 of experience. The only way is to take readings several times 

 and to strike an average for the result. It will, probably, be 

 found that if four readings be taken, the Apertometer pointers 

 will be at a ditferent position on each occasion. 



After many years of exi)erience in examining and criticising 

 lenses, I have found nothing so thoroughly satisfactory for 

 getting at the real accuracy of an objective as the " Abbe Test 

 Plate.' Its merits are hardly recognised in the miinner they 

 should be probably for the reason that microscopists generally 

 are quite, content to take the quality of their objectives for 

 granted and to refrain from splitting hairs over fine details, but 

 no man can be a master of his instrument without being fully 

 conversant with the possibilities or drawbacks which his lenses 

 may possess. (Tu he ronlinued.) 



New MiciiOSCOPE?. — Messrs. R. & J. Beck have sent a price- 

 list and particulars of the "Imperial" Jlicroscope, a new 

 instrument di signed by them for critical work. 



It is m,ade in four different models and supplied with both 

 horseshoe and tripod bases. The most complete instrument of 

 the series on tripod foot has special claims to consideration, for 

 it is provided with every convenience that the modern worker 

 with objectives and condenser of high aperture could desire. 



All the instruments of the series are fitted with the new two 

 s|>eed fine adjustment to which reference was made in these 

 columns in the .Tauuary number. 



I am just a little doubtful of the expediency of fitting a 

 mechanical stage with the controlling milled beads perpendicular 

 to its surface. In turning them, pressure will be exerted 

 downwards and there would be a natural tendency for the 

 object to go out of focus. 



By adopting this method of construction, however, the added 

 convenience of complete rotation to the stage is gained and the 

 plates of the mechanical fittings do not come in contact with 

 the substage condenser, a very desirable consummation. 



FoRAMisiFKRors Material. — The number of ap()licants for 

 this so far exceeded expectations, that the supply to each one 

 had to be limited considerably. Several readers omitted to send 

 a stamped directed label ; a small quantity may yet be had by 

 such on compliance with this condition. 



M<>i:xTi.si; AM) PRF.rARiNi; Eotifehs. — For the benefit of 

 those readers who may wish to experiment on the lines recom- 

 mended by Mr. Rousselet, and who may have difficulty in 

 securing the specimens, it may be worth while to mention that 

 both Mr. Bolton, 25, Balsall Heath Road, Birmingham, and 

 5Ir. Pears.all, Headmaster, Senior Jlixed Board School, Dalton- 

 in-Furness, supply Rotifera in tubes : the former at Is., and the 

 latter at Is. lid. per tube including postage. 



The Rev. E. J. Hollowaj-, of Clehonger Vicarage, Hereford, 

 also kindly offers to fupply Rotifer material, particularly 

 jiliihiiliixi, to readers, in order to further the work of the 

 ■'practical'' scheme. It would be courteous if such as avail 

 themselves of this offer would send a tube with an addressed 

 label bearing stamps for the cost of the return post.ige. 



A lady reader also kindly offers to supply a limited quantity 

 of ilycetozoa. 



CoNSVLTANTS. — In respoDso to the invitation for assistance in 

 the January number, one gentleman has kindly consented to 

 name Diatoms and another to identify micro fungi infesting 

 plants, etc. ; if any who feel able to assist in other departments 

 will communicate with me, I shall bo glad. 



NOTES AND QUERIES. 



0. Ei-diix. — The material concerning which you require 

 information has been kindly examined by Mr. A. Earland, 

 together with your drawing, and the following is his report : — 

 "True coccoliths, /.c, the calcareous surface plates of the micro- 

 scopic alga Coccospliaera have been found in the chalk and in 

 other fossil deposits, but they are of rare occurrence." 



The term " coccolith '' is, however, very loosely used by 

 geologists for the descri|)tion of almost any small plate-like 

 perforated body, approximating to the true coccoliths in shape 

 and size. 



;Mr. Oswald Evans' figures 1—8 and 10, with one possible 

 exception (fig. 9), all belong to well-known forms of these 

 "spurious"' coccoliths, which are of frequent occurrence in many 

 fossil deposits. Their nature and origin are quite unknown. One 

 thing only is certain, they have no connection with the true 

 coccoliths, which have au altogether different structure. Pre- 

 sumably they are the remains of some organism or organisms 

 quite extinct and unknown. 



The essential feature of a true coccolith is its " double 

 structure.' Each coccolith con.sists oE two curved plates (oval 

 in Cm-roxpliiiera /ir/nr/iiu Wallicb, and round in C. Ipplnponi 

 Murray and Blackman) united by a centnai column, which is 

 pierced by a median pore. A true coccolith therefore bears 

 some resemblance to a shirt stud with a hole pierced right 

 through the middle. 



Figure 9, though it does not show this double structure, bears 

 some resemblance to a true coccolith (of C. leptopora), and 

 might prove to be one. Figures 11 and 12 are rather curious; 

 11 seems to be a stellate microschlere from a sponge ; 12 might 

 be of Radiolarian origin ; 19, 20, 21, are certainly sponge 

 spicules, and 22 is probably a sponge sterraster with adventitious 

 perforations. 



Rhabdolith is used by geologists in a very wide sense also. 

 Of the Figures KJ — 18, only two, viz., 15 and 1(3, rotdd be true 

 rhabdoliths, and the detail of the drawing is not sufficient to 

 determine this with certaintj'. 



Figs. 13, 14, 17 and 18 are almost certainly of sponge origin, 

 more or less eroded. 



J. Searlf, Opurlfi — This correspondent has expressed his 

 willingness to supply material towards the success of the prac- 

 tical scheme. He suggests that living in Portugal, there might 

 be some object or series of objects which would be interesting 

 to readers. Also he would be glad to exchange material, etc., 

 with other microscopists. If any readers would like to com- 

 municate with this gentleman, I shall be pleased to re-address 

 their envelopes. 



F. R. C. S. — Any of the binocular microscopes made by the 

 opticians who advertise in this journal would give you 

 satisfaction. You will find that it is well not to have the 

 binocular body less than 8 ins. long ; shorter ones have 

 been made, but they are not ne.arly so satisfactory. The 

 Wenham, which is the usual kind of binocular, will prove most 

 serviceable : it can alw.ays be rendered monocular by sliding the 

 prism out of position, for which provision is made ; the light 

 then only passes up the monocular tube. Binocular eye pieces 

 have never proved really comfortable in the tests that I have 

 made. 



t/. //. G. inquires if any method is known of mounting 

 Cladocera so as to preserve the transparency. He al-so suggests 

 that some notes on the preparation and mounting of Entomos- 

 traca might prove interesting. Can any readers give information 

 or assist in these matters V 



G. Fisk. — Seeing that you have a |" objective, it is absolutely 

 necessary for you to use an illuminating apparatus, you cannot 

 secure satisf.actory results without it. It its absence you are 

 not working your objectives to anything like their full capacity, 

 and you will be surprised at the improvement you will obtain 

 when the condenser is added. 



T. Vclerx. — An article on Microtomes as suggested by you will 

 appear in a month or two. 



