July 1, 1899.] 



KNOWLEDGE. 



165 



not diminished our wonder and admiration of this marvel 

 of minute stracture. But in the vast majority of cases, if 

 a microscopist is asked to show the teeth, a denial of their 

 existence would be received, probably coupled with an 

 assurance (as I have been assured) that there were no teeth, 

 but some minute hairs on the edge of the labella which 

 had been mistal;en for them. It is a curiosity of micro- 

 scopy that often, unless it is known what to look for, it is 

 quite possible to miss seeing a structure or detail, from the 

 fact that its focus is at a different distance from some near, 

 yet more prominent object. This is precisely the reason, 

 coupled with the fact, that the proboscis is usually looked 

 at with low powers, why these teeth escape observers; they 

 lie at the base of the false or pseudo trachefe, at a different 

 focus, and the chitine of which they are composed being a 

 light yellow and transparent, they are easily overlooked. 



Like the teeth of the Dung Fly {ScdtopJuvja), they were 

 discovered by Mr. W. H. Harris, and figured in Science 

 Gossip, in 1885, but the discovery seems not to have 

 become known out of a small circle— as it is mentioned as 

 fresh in the Journal of the Eoyal Microscopical Societi/, in 

 July, 1898. 



The teeth show between eleven of the false tracheas, and 

 six appear to be double, making a total of sixteen on each 

 labeUum. With the exception of one at the end furthest 

 from the head, they have a cleft edge, thus securing two 

 points to each tooth, and, one might suppose from this, are 

 used for pricking or gently scraping a surface, possibly 

 with the object of accelerating a flow of liquid. They are 

 much thinner than those of the Scatopharja, and not round 



Teeth of Blow fly, magnilied 170 diameters. 



and tooth-like, but riband-like in appearance. The bases 

 appear slightly rounded, and from these the teeth broaden 

 and taper to a very thin edge — they might be compared to 

 a row of chisels with cleft edges. 



To see them a magnification of three hundred and fifty 

 diameters is quite sufficient — a good quarter-inch power 

 would do ; the field at the back of the teeth is rather 

 obscured by the chiiinous support of the labellum, but if 

 the bases of the false trachete are fooussed, the characteristic 

 edge of a tooth is probably seen, and that once found, the 

 rest is easy. The length of the row of teeth is three two- 



hundredths of an inch. 



♦ 



By JOHX H. COOKE, F.L.S., F.G.S. 

 The embedding of soft tissues requires considerable skill on 

 the part of the operator. But frequently, notwithstanding the 



greatest care, failure results, either owing to the variable nature 

 of the medium, used, or because the processes of hardening have 

 been too protracted. Those who have experienced these 

 difficulties should try the white of an egg as the embedding 

 medium. The process is as follows :— JIake a small paper box, 

 about a half-an-:nch square, and 611 it with the white of an egg. 

 Eliminate all alcohol from the material which is to be mounted, 

 and embed it in the albumen. Expose the box and its contents 

 to heat, and when hardened place it again in alcohol. If the 

 sections are passed through oU of cloves with balsam, the albu- 

 men will become clear and transparent. 



An illustration of the pitfalls that confront science students 

 is given by Dr. Martin Ficker, a German bacteriologist. 

 Among various sources of error in work with bacteria is the 

 glass of the vessel used, as different kinds of glass impart vary- 

 ing degress of alkahnity to water, and it is found that some 

 bacteria, notably those of cholera, are favourably affected by 

 alkalinity. This novel cause seems responsible for marked and 

 important discrepancies. , 



Freshwater mites do not, as a rule, make satisfactory mounts. 

 They shrink and fade, thus losing in the preserved state the 

 beautiful symmetry and colourings which render them, when 

 ahve, such charming objects for observation and study. The 

 ditiicnlty is the mounting medium. Xo formula is known 

 which wiU give perfectly satisfactory results, but the following, 

 if carefully prepared, wiU enable the microscopist to preserve 

 his specimens, for some years at least, from bleaching and 

 collapse : — Prepare three mixtures of distilled water and pure 

 "Ivcerine in the proportions of twelve parts, ten parts, and 

 eight parts of water, respectively, to one part of pure glycerine, 

 and to the last add a small drop of carbohc acid. Place the 

 specimens in the twelve-part mixture and leave them for twelve 

 hours, after which place them in the ten-part mixture and leave 

 them for a similar period. They may now be permanently 

 mounted in the third mixture. SoUd glass cells are preferable 

 to built-up ones. 



The preservation of labels of reagent bottles is a frequent 

 source of trouble to the working microscopist. A good varnish 

 for this purpose may be made by macerating the following 

 substances, and thoroughly shaking the mixture until all are 

 dissolved : — Sandarac, sixty parts ; mastic, twenty-five parts ; 

 camphor, one part ; oU of lavender, eight parts : Venice 

 turpentine, fotu- parts ; ether, six parts ; and alcohol, forty-four 

 parts. 



It is frequently desirable to supplement the records obtained 

 by photographing micro-sections with a drawing, and it would 

 probably be oftener done were it not for the difficulty that 

 many experience when using either the neutral tint reflector or 

 the camera lucida. With a vertical camera and suitable tracing 

 paper this difficulty disappears, and it is possible, after a httle 

 [jractice, to acquire a creditable amount of dexterity in repre- 

 senting the salaent features of all objects that do not require 

 high-power objectives for their delineation. In making such 

 drawings it is a great advantage to be able to do them direct 

 from the microscope, so that no transference or copying is 

 afterwards needed. For this purpose, the following recipe for 

 making a tracing paper, which can be reconverted into ordinary 

 di-awing paper after the drawing has been made on it, will be 

 found to be both simple and effective : — Immerse any ordinary 

 drawing paper in a mixture consisting of one volume of castor 

 oil and three volumes of spirits of wine, and hang it up to dry 

 in a warm room for two or three hours. Place a sheet in the 

 focussing screen of the camera and make the drawing, after 

 which place it in a bath of spirits of wine and allow it to remain 

 there untH the oil has been dissolved out. The paper will 

 resume its former state and appearance. 



The isolation of the skeletons of siliceous organisms, foramens, 

 and other small objects from rock specimens, may be readily 

 effected by first drying the rock in air and then dropping it 

 into a hot, saturated solution of Glauber's salts. On coohng, 

 the processes of crystalhzation break up the rock mass. 



Balsam mounts are apt to deteriorate, unless some means are 

 adopted to prevent the access of air or moisture. They may be 

 protected by running a ring of melted paraffin wax around the 



