92 



KNOWLEDGE, 



[April, 1903. 



will be found especially useful in this process, as the lenses 

 form a good screen for the eyes from the otherwise too pungent 

 fumes of the formalin. 



It occurred to me that possibly the formalin might injuriously 

 ailect the delicate cell structure of the moss leaves, but 

 experiment assured me that this need not be feared, and I began 

 to hope that here, at least, was a method of mounting that 

 would practically eliminate all serious risk of failure. My 

 dis;ippo!ntment was consequently great when, on going to my 

 cabinet a couple of months ago, I found that several of the 

 slides mounted with formalin had gone wrong. What puzzles 

 me all the more is that slides in the same drawer, put up some 

 with formalin and some in the ordinary way, show no sign of 

 change. Thus slides of H>/piium, Com inula turn and Amlrcrra 

 alpin<i mounted five or six years ago give no indication whatever 

 of deterioration, while others, completed fully twelve months 

 f-ince, and which have purposelj' been allowed to stand in a room 

 without a tire, are still in a perfectly satisfactory condition. 

 Moreover, in January, VM^i, I put a few drops of the jelly on 

 a glass slide, and mixed with them a little of the formalin 

 solution : this slide has also laid in a room without a fire, and 

 the jelly, though unprotected by a cover-glass, is still compact 

 and firm. The drawer containing the specimens in question 

 happens to be the one nearest" to the ground, and the cabinet 

 stands in a corner of my dining-room furthest removed from the 

 likelihood of draught, but as the door of the cabinet fits closely, 

 it is difficult to believe that change of temperature constitutes 

 the explanation, especially as the slides in the drawer not far 

 above the one in question are unaffected. I have recently been 

 making still further experiments, though sufficient time has not 

 yet elapsed for any definite conclusions to be drawn from them ; 

 on the whole, however, I am inclined to think that too much 

 formalin was used in the mounts referred to, and that herein 

 lies the cause of their failure. Of late, therefore, I have dipped 

 little beyond the extreme point of the glass rod into the 

 formalin solution, thus transferring a much smaller quantity to 

 the jelly in the cell ; this will not, of course, solidify the jelly 

 so thoroughly as if the larger quantity were used, but, on the 

 other hand, it will not have the same contractive power. 



In concluding these notes, let me again emphasise what I said 

 at the outset, namely that they have been written in the hope 

 that others may thereby be induced to give the results of their 

 experiments in the matters of which they treat, a remark which 

 specially applies to the somewhat full detaOs given with regard 

 to the use of formalin. I am well aware that much that they 

 contain will certainly not be fresh to many workers, yet 

 possibly for some who have not long taken up the study, there 

 may be in them a few practical hints which will be of service. 



THE MOUTH PARTS OF THE TSETSE FLY. 

 By W. Wesch£, p.r.m.s. 



It cannot be denied that it would be difficult, if not impossible, 

 to find a more destructive instrument than the proboscis of the 

 Tsetse fly {Glos^ina). Yearly it causes the death of large num- 

 bers of horses and cattle, and whole districts, one might say 

 whole provinces, are depopulated on account of its presence. 

 Yet the part itself is innocent and interesting ; were it not 

 for the parasitic habits of certain lowly organisms it would be 

 quite innocuous. We see this clearly enough when we examine 

 our English relatives of the Tsetse fly, Stotnoxi/s calcitrans, 

 Hamatobia atimulans, H. irritans and Prosena sybarita. 



Apart from its character as an organ of destruction the 

 proboscis of Tsetse has interest ; we can trace its development 

 through several stages back to a trophi similar to that on our com- 

 mon larger house fly. In its case, as in all, the rule of the French 

 navant Savigni holds good, " the organ is the same in all insects, 

 it is only the use that is clianged or modified." By examination 

 of the structures and parts in many species it appears that Tsetse 

 as a species is a comparatively recent addition to our fauna, 

 which has developed a peculiar mouth part for a special purpose. 

 In our larger house fly {Musca dumextica) the proboscis is a 

 suctorial organ, with the ordinary characters of such a mouth 

 part as found in Di|)tera, that is to say, the outer and inner jaws 

 (the mandibles and maxillae) which we find in the gnats 

 (Culicidie). the breeze flies (Tabanidae), and other biting insects, 

 have been absorbed into the lower lip (luhiumj, and now act as 

 stiffemng rods and levers to support the two suctorial discs 



(para(]lossm) at the extremity of the lower lip. The discs or 

 labella are fitted with symmetrical rows of tracha;, or tubes 

 formed of chitinous rings. 



The lower lip has on its under or ventral surface a chitinous 

 place (the mentum) which forms a support. Above the lower 

 lip is the labi-iim or upper lip, which has, however, its base 

 enveloped by the membrane of the lower lip. The upper lip 

 in Diptera is a horny lancet case, which covers a lancet (the 

 hi/j>o/>Iiari/)ix), which corresponds with the linr/ua or tongue. 



The upper lip is furnished with two feeling organs (the /«i/a/ 

 palpi). Till latel}' these were thought to belong to the inner 

 pair of jaws {maxillif), but the discovery of complete palpi on 

 many species enables me to say with certainty that these are 

 labial, or belonging to the lower lij). 



( To be coiitinueil.) 



MONOCHROMATIC LIGHT (Continued). 



Beyond this, and of greater importance, is the equivalent 

 effect which can he secured with a good achromatic lens — equal, 

 in fact, to that with an apochromatic. All residual colour in 

 the lens is absorbed by the one colour of the light filter, so that 

 the great difference between the chromatic and apochromatic 

 sy.stems, namely, the outstanding secondary spectrum, is thereby 

 abolished. 



If all the colours of the spectrum are used in their proper 

 succession to illuminate a given object, it will probably be found 

 that a re-focussing is necessary for many of them, the greatest 

 amount of variation being noticeable usually at the two ends of 

 the spectrum. 



Jlr. Xelfon some years ago prepared a series of charts in 

 which this subject was exemplified with a varietj'of lenses. In 

 every lens the point of least chromatic aberration is indicated by 

 the focal point remainiug practically the .same for a continuous 

 range of colour rays. This same point will be the one of least 

 spherical aberration also if the optician has done his best. If, 

 now, a colour screen is selected which approximates to the mean 

 of the portion of the spectrum which exhibits the least difference 

 of focus, and therefore the smallest amount of spherical aberra- 

 tion, the best result would be obtained with the objective. 



Before giving formuhc for making fluid monochromatic light 

 screens, the method adopted by Dr. Spitta for obtaining his 

 monochromatic light, as exhibited at the Royal Microscopical 

 Society recently, may be of interest. The following was the 

 system used in the order of arrangement : — 



1. Oxy-hydrogen limelight ; 



2. A condenser composed of two i>lano-convex lenses, 



liaving placed between them a water bath : 

 :l A slit ; 



4. An achromatic coUimating lens of about 6" focus, com- 

 posed of a double convex and plano-concave lenses 

 cemented ; 



5. A prism, having mounted on one of its faces Thorpe's 

 grating, so arranged as to obtain direct vision for the 

 F line ; 



G. A concave lens immediately behind a substage con- 

 denser, the object of which is to render the rays 

 divergent, so as to suit the flame distance for which 

 the substage condenser is corrected ; 



7. A holoscopic oil-immersion condenser. 



Fluid Screens. — One of the most useful screens is that 

 which is known as " Giffoid's,'' and is made by dissolving a few 

 crystals of malachite green in gljcerine. The solution is 

 examined spectroscopically with the illuminant that is to be 

 employed, and green is slowly added until the red end of the 

 spectrum is entirely absorbed. It should be noted that a piece 

 of signal green glass in conjunction with this solution materially 

 reduces the thickness of the trough that would otherwise be 

 required. (To be rontinued.) 



'^THE JOURNAL OF APPLIED MICROSCOPY AND 

 LABORATORY METHODS." 



For five years past there has been published at Rochester, New 

 York, U.S.A., a journal with the above title. -^s this title 

 implies, it is essentially technical and ])ractical. 



I have before me a bound volume for the year 1902, and the 

 ground covered by the various articles is very wide and full of 

 interest for microscopists. 



