Septejcber, 1902.] 



KNOWLEDGE, 



213 



Conducted by M. I.Cro§§. 



PoxD-i.iKE Coi,i.i;CTiN(; IN Skptemher.— Ill normal years 

 many of the drieil-up pomls begin to Kll up again in September, 

 and become then most prolific in Infusorian and Rotiferous life, 

 because the disturbing Crustaceans, Cyclops and Cladocera, 

 have been to a large extent eliminated. But also in larger 

 ponds and lakes, which do not dry up, the Crustaceans decrease 

 in numbers and give the Rotifera and Infusoria a fresh chance 

 of increase. The following free-swimming forms m.iy often be 

 collected in immense numbers by means of the (^hieketter's 

 collecting net and bottle : AspUinrhna priodoiHa, iiitermedia and 

 hriiililiiel/i ; Triarllini Ittngiteta : Poli/dithra ])l<iti/j)le.r(i ; 

 Si/ncliiwta pectiiiata, Iri'iiiulu and <:h/iiiif/ii : Aiiurnd aruleiita 

 ntul fochteari'i ; Hidchiunus angularis : Peilnlion minim ; Coiio- 

 cliihix unicornis ; and the much rarer Floscularia pelvi/ica. Of 

 the fixed forms Limiiia-s reratoplnjlli and anmdutus, Cephalo- 

 siphon limnia.':, Lacinidaria socialix, .Uelicertii r/ni/i'n.f and 

 r-mi/era should be looked for on submerged water plants, such 

 as Anacharis, Ceratophyllura, Nitella, and on the rootlets of 

 Duckweed. Polyzoa such as Plumatella, Lophopus, Cristatella, 

 should be found in abundance in disused canals and backwaters 

 of rivers and the larger lakes, from which they should be 

 dredged with a loaded hook and line. 



It may be taken as a general rule that all the more interesting 

 forms of Pond-life become more abundant in September, pro- 

 vided only the weather is not too hot, but tempered by rejieated 

 showers to fill the dried-up ponds with a fresh supply of rain 

 water. 



Microtomes. — The selection of a microtome is largely 

 dependent on the description and quality of the work that it is 

 proposed to do. The majority of amateurs find their aims 

 SJitisfied with the ordinary well microtomes which are made in 

 numerous forms : and for those who do occasional work, and 

 that not of the finest kind, in which sections of extreme tenuity 

 would be essential, these are undoubtedly good and satisfactory. 



Further, ability to cut sections of a certain kind with them 

 is easily attained ; but to do the best that is possible with them 

 requires as much practice and experience as is necessary for the 

 acquirement of facility in any other line. 



There is, however, a limit to the capabilities of these micro- 

 tomes even in the hands of the skilful and trained manipulator, 

 and it is beyond this that resort has to be made to one of the 

 more complex forms of microtomes, of which so little is known 

 outside laboratories. It is proposed therefore, in a brief way, 

 to mention some of the leading forms of microtomes, and their 

 special features. 



Of English make there are but two microtomes that have a 

 right to be reckoned as of the first rank ; one is the well-known 

 Cambridge Rocking Alicrotome, made by the Scientific Instru- 

 ment Co., of Cambridge, and the other is the " Uelepine,' made 

 by Messrs. R. & J. Beck. The former is designed especially 

 for the cutting of tissues of small size embedded in paraftin 

 wax, aud for that particular purpose it has probably no superiors, 

 and when once its ]ieculiarilies and ])ossibi)ities have been 

 thoroughly gi'asped, it is capable of work of the highest class. 



The " Delepine " Microtome, which has been made by Jfessrs. 

 Beck to the suggestions and designs of Professor Delepine, is 

 intended specially for the cutting of frozen tissues ; it is of 

 substantial build and excellent design. 



Beyond these, it is necessary to go to the Continent for our 

 wants to be supplied, and all of the makers, which are very 

 limited in number, make instruments which have special points 

 of value for different workers. Some of them are famihar to 

 the ordinary microscopist through their inclusion in English 

 microscopic catalogues, but there are otliers which are rarely 

 heard of, and are deserving of consideration. 



One of the cheapest microtomes tluit can be considered of a 

 practical description is that made by Schaiize, of Leipzig. The 

 jaws will hold an object IJ inches by J inch, and the object can 

 be orientated and fixed in any position. The knife-holder guides 

 consist of three raised rails 7 inches long. Provision is made 

 for the measurement of the thickness of the section by means 

 of the micrometer screw, which is of good construction. The 

 price of this complete is Sits. 



Next to this is an exceedingly efficient instrument by Jung, 

 of Heidelberg (his catalogue No. 11'.)), which is provided with 

 an automatic raLsing arrangement to the object holder of very 

 ingenious description, the knife being brought into action by a 

 swinging movement. It c^in be used both for embedding and 

 freezing. The price of this in case complete is oOs. 



The same maker is res]ionsible for the Rivet Model Microtome, 

 which is considered one of the most generally useful and best 

 machines by a large numlier of workers. In this, the knife 

 moves in a horizontal plane, and the object in an inclined plane. 

 The accompanying figure gives a diagrammatical view in section 

 of this microtome. 



D is the base of the microtome. 



C is the central support. 



The V guides formed by AC and BC have projecting friction 

 surfaces ; while the two blocks O and K, the former the object 

 carrier, the latter for the knife, have fine ivory points where 

 they make contact with the guides. 



The Y fitting BC is inclined one in twenty, and, obviously, as 

 the block O is caused to move along the groove, the object is 

 raised. A suitable vernier is fitted, which gives exceedingly fine 

 readings for the travelling imparted, which may be effected 

 either by micrometer-screw or hand. 



(To he continued.) 



Measuring Apertures of Oimectives.— It is generally 

 supposed that the only suitable means obtainable for measuring 

 the apertures of objectives is the apertometer made by Zeiss. 

 Truly, this does work efficiently, yet even with it, marked 

 discrepancies frequently occur between the results obtained by 

 independent observers, and it seems to be pecuharly difficult to 

 secure uniformity, especially with dry objectives having large 

 back lenses and oil immersion lenses. Accuracy with this, as 

 with many other instruments, greatly depends on experience 

 aud practice. 



The apertometer itself would be so very rarely used by the 

 ordinary worker that it is not considered a necessary adjunct, 

 added to which its expense is in itself an obstacle. This is 

 pointed out in an extremely useful paper by Mr. H. F. Angus, 

 which appears in the ,/«»/■»<// of the Quekett Microscopical Club 

 for April, VM2 : and he further i>roceeds to suggest ways and 

 means for ascertaining the apertures of dry objectives with very 

 simple means. 



The most practical is probably the protractor apertometer, 

 which consists essentially of an ordinary semi-circular protractor 

 mounted vertically on a base with its diametrical edge towards 

 the objective, and having attached to its circumference two 



