May, 1908.] 



KNOWLEDGE 



117 



there can be little doubt that the development has been on 

 similar lines, though P. aybarita has the trachae still showing at 

 the end. This is a stage through which Glosnina undoubtedly 

 passed before it developed the chitinoua ridges that now 

 characterise it (Fig. G). The large swelling at the base of the 

 proboscis was for some time thought to be the receptacle for 

 the Tsetse poison. This was plausible, but has no foundation in 

 fact, as this part is simply full of the complicated muscles 

 necessary for the working of the proboscis. The herptomonads 

 which are the pathological element are an accident, and even 

 an unfortunate accident regarded from the Tsetse point of 

 view. They form part of the food of the insect, sucked with 

 the blood from the antelopes and zebras, and may be found in 

 any part of the alimentary canal. Till the Tsetse has sucked 

 the blood of an infected animal it is like the Anopheles 

 mosquito in a similar state, perfectly harmless. 



I have been asked to s.ay where specimens of Tsetse can be 

 obtained. I am afraid that they are very difficult to come 

 by. The only chance is through friends in Africa, and that 

 chance but a remote one, as man avoids the Tsetse country as 

 much as possible. Even if the flies can be caught it requires a 

 certain measure of skill to set them properly, and they will 

 probably arrive in a very unsatisfactory state. 



In the Natural History Museum at Cromwell Road is one 

 of the earliest specimens obtained. This was sent home by 

 Dr. Livingstone, and is simply the insect crushed on a piece of 

 paper. This is cherished as a relic of the great explorer, but is 

 a sorrowful object to an entomologist. I have also seen some 

 that another great traveller. Captain Speke, collected ; these 

 were pro]ierly pinned and set, and, considering how many years 

 they have been in the Museum, are in fairly good condition. 



The figures are drawn from preparations, cleared and mounted 



with pressure. ■-.-. 



"THE HELMHOLTZ THEORY OF THE MICROSCOPE." 



The meeting of the Royal Microscopical Society on March 

 18th was devoted to a paper by Mr. J. W. Gordon on " The 

 Helmholtz Theory of the Microscope." Mr. Gordon's paper 

 was sugges-ted by an article which Helmholtz contributed in 

 1874 to Pot/!/en(luifs Annalen under the title, ",The Theoretical 

 Limit of Resolving Power in the Microscope '' ; " the merit of 

 which '' — in Mr. Gordon's opinion — " lies not in this formal 

 result (that is, the demonstration of a definite limit of resolving 

 power), which is, in fact, not successfully established, but in the 

 line of investigation which Helmholtz here strikes out, and 

 without following it to its practical issues, pursues far enough 

 to present his readers with a surfeit of interesting and valuable 

 suggestions. These suggestions are, many of them, so obscure 

 and conveyed by such subtle hints that they may well escape 

 attention, and apparently they have escaped attention to the 

 present day." 



Mr. Gordon attempted the extremely difficult task of giving 

 an account of Helmholtz's paper without " its somewhat 

 repulsive mathematical garb, " a task the more difficult in the 

 present case because Helmholtz himself, in a postscript, frankly 

 acknowledging the priority of Prof. Abbe with regard to the 

 enunciation of the several theorems contained in his paper, 

 suggests as the latter's only scientific justification just the 

 mathematical proofs of those theorems which Abbe had not then 

 published. 



Briefly stated, Mr. Gordon's paper gives first a non-mathe- 

 matical dissertation on the theory of diffraction and difi'raction 

 gratings, which, he says, Helmholtz takes for granted, and 

 writes as abstrusely about as the most hardened mathematician. 

 Then follows a long section on the famous Sine-condition, 

 which has probably suffered most — as compared with Helmholtz's 

 lucid i)apor — by the etfort to suppress mathematics. Finally 

 the contraction of the emergent pencils with increasing magni- 

 fying power is discussed as affecting the brightness of the 

 image, the obtrusiveness of entoptic appearances, and especially 

 the rapid wi<iening of diffraction fringes. 



It is in this last section that Mr. Gordon finds fault with 

 Helmholtz's reasoning and endeavours to prove him in the wrong. 

 It is, however, to be feared that not many scientifically educated 

 microscopists will prefer Mr. Gordon's arguments to those of 

 the great physicist. 



Having convinced himself that Helmholtz's reasoning as to the 

 limit of resolution is quite wrong, Mr. Gordon once more tries 

 to suggest improvements of the microscope which would or 



might extend its powers. It will be remembered that about 

 two years ago, in a paper read before the Royal Microscopical 

 Society, Mr. Gordon suggested that the " antipoints " might be 

 made as small as desired, and the definition accordingly improved, 

 by some arrangement of lenses behind the objective. This time 

 we are offered an oscillating screen. The image projected by 

 the objective is to be focussed on a finely ground glass-screen, 

 which is, moreover, to be set oscillating in order to render its 

 grain invisible ; and the image so projected is to be viewed by 

 a second microscope so as to get a high magnification without 

 an excessively contracted emerging pencil. 



In a first appendix, Mr. Gordon gives what he believes to be 

 a proof of the .Sine-law, or, rather, of a Sine-tangent law : he 

 also endeavours to show that Hockin's elegant proof — as accepted 

 by Prof. Sir Silvanus Thompson, Dr. Czapski, and other autho- 

 rities — is wrong. In a second appendi.x he tries to prove by a 

 kind of graphical integration that lines much closer than would 

 follow from Helmholtz's formula can be resolved. 



A third note tries to prove Helmholtz in the wrong as regards 

 the impossibility of suppressing diffraction-phenomena ; and a 

 fourth points out an error in a published translation of 

 Helmholtz's paper. 



The late hour at which the reading of the paper was con- 

 cluded unfortunately cut short what might evidently have been 

 an interesting and instructive discussion. Still, it sufficed to 

 show a strong conviction in certain quarters that Mr. Gordon 

 had over-estimated the importance of " the Helmholtz theory,'' 

 and h.ad, at the same time, not done justice to the lucid reasoning 

 of the original paper. 



Mr. Gordon's interpretation of the Sinelaw was particularly 

 objected to (he maintained that it applied to objects and images 

 of sensible size, whereas it is obviously, as a strict mathematical 

 theorem, limited to a surface element in the optical axis), and it 

 was pointed out that he had not given Helmholtz's proof of the 

 Sine-law proper at all, which Helmholtz deduced by integration 

 from a more general one applying to all centred optical systems, 

 but restricted to small angles of divergence and of incidence. 

 Considerable doubt was also expressed whether the oscillating 

 screen would put any detail into the image which could not be 

 seen with a suitable eyepiece. 



A careful perusal of the paper and a comparison with the 

 original German paper of which it is meant to give an account 

 must convince anyone familiar with optical and microscopical 

 theory that it bristles with statements which invite criticism. 



MONOCHROMATIC LIGHT {Continued). 



The following list of light filters, any of which can be easily 

 made, and will be found invaluable in photo-micrography 

 especially, have been tried in actual working. They are from 

 the formulsB of Dr. Nagel, of Freiburg, and with others of a 

 similar nature are detailed in " Biological Laboratory 

 Methods," by Mell. 



OR.vxciE Filter for the spectrum district between C and D. — 

 Prepare a solution of acetate of copper, add a few drops of 

 acetic acid, and then, drop by drop, a concentrated saflEranin 

 solution, until the solution will admit no more violet, blue, 

 green and yellow light. 



Yeli.()\v-ray Fii.trr. — Add to a saturated acid solution of 

 acetate of copper a saturated acidified solution " Orange G." 

 The liquid has a brown appearance, and passes only a small 

 stripe of yellow light. 



Green-yei.i.ow Filter. — To a saturated solution of bichro- 

 mate of potassium acidified with acetic acid, add crystals of 

 acetate of copper, and heat the solution. The green liquid 

 passes only monochrome green light. 



Blue-ray Filter.— A weak solution of methyl-gi-een is 

 mixed with acetate of copper .solution until no red light passes. 



Reference was made in the last number to theGifford Screen 

 for line F, and the special qualities of the acetate of copper 

 screen, which has beeu previously described in these columns, 

 should not be overlooked. This screen consists of a saturated 

 solution of acetate of copper. The solution is placed in a 

 trough which should have an interval between its back and 

 front glasses of at least |-inch. |-inch exactly absorbs the 

 red end of the spectrum when an oil lamp having a ^-inch 

 wick is used. If a more brilliant illuminant be emplo3'ed 

 an increased width of trough might bo also used without 

 disadvantage. 



