Jan'uary, 1911. 



KNOWLEDGE. 



13 



ibsence of rats it 



.ill 



dil 



animals. In the 

 bite man. 



It will be seen at once how important this factor 

 is. The var\-ing appetite of the different kinds of 

 fleas modifies to a great extent the probabilit\- of 

 the transmission of plagne to man. If it were 

 possible, or probable, that the European rat flea 



should alter its habits and adopt man as a host, 

 the probabilit\' of human plague in Europe would 

 be of an entireh" different order. At present 

 northern Europe, and, to a less extent, Europe in 

 the temperate zone, seems immune from an out- 

 break of plague on account of the habits both 

 of the grey rat and of the flea which lives on it. 



CORRESPONDENCE. 



PODL'R.4 SC.-'iLES. 



To the Editors of " Kxowledgi;." 



Sirs, — While fully appreciating Mr. I'lasUitt's kind rufer- 

 ence to my article, for which I thank him, there are yet 

 certain points in liis letter to yourselves witli which I, for one, 

 cannot agree. I cannot admit, for instance, tliat in the 

 Podura scale mounted dry we are " left with the only alter- 

 native (that is, in being mounted in a medium) between the 

 normal of air and the scale itself"; or, that for that reason, 

 " we make no advance upon the earliest methods." .'\s I 

 understand it, with a scale mounted dry. and tight against the 

 cover glass, there is no normal of air, but perfect optical 

 contact with glass throughout, also with the cedar oil of the 

 same refractive index between that and the objective. Mr. 

 I'laskitt speaks of the scale, not being flat, giving rise to 

 distortion. That has not been my experience, though it may 

 be said of other kinds of scales, such as those belonging to the 

 Moth and Butterfly. 



Assuming that the widest apertured oil immersion objective 

 is employed upon this scale, the best way to discover whether 

 it is working at its maximum is to look down the tube of the 

 microscope after removing the eye-piece. If the back lens is 

 seen then to be filled as to tliree-quarters of its diameter with 

 white light, this condition may be said to be fulfilled. Under 

 no circumstances could this be obtained with an objective of 

 r40 N.A. were there a stratum of air between the object and 

 the cover glass. 



These conditions were fulfilled in all my work upon this 

 scale. For myself I never required other proof of optical 

 contact, than the flashing out of the object in brilliant light, as 

 if the scale were self-luminous. Apart from this, however, 

 my friend, Mr. E. M. Nelson, with whom, at that time, I spent 

 many delightful days at his house over the microscope, tested 

 it for his own satisfaction with the Vertical lUuniinator, when 

 the test was always justified. 



As to the three-quarter cone of white light at the back lens 

 of a wide apertured objective; this for a long time now has been 

 looked upon as the ideal illumination, where the greatest resolv- 

 ing power is combined with perfect definition. Mr. Nelson, in 

 his Presidential Address to the Quekett Club in the early part of 

 1895, went fully into this cjuestion. The address will be found 

 in Vol. 6 of the Journal of that Club, beginning at page 14, but 

 as some readers may not have access to it, I beg to be 

 allowed to make the following extract for their benefit : — 



" It is common knowledge that when a full cone is employed, 

 the resolving power falls off, and it has been customary to 

 account for this falling oft" in the resolving power by the 

 outstanding spherical aberration in the objective. To test the 

 accuracy of this current notion a critical image was set up, 

 and matters arranged so that access could be obtained to 

 the back lens of the objective without disturbing any of 

 the adjustments. When a full cone of light was used (that 

 is, the back lens entirely filled with light — T.F.S.) the 

 resolving power fell oft, and when a three-quarter cone 



was employed it was as usual restored again ; a stop 

 was then placed at the back lens, cutting off the 

 peripheral unilluminated annulus. We had, therefore, an 

 objective of less aperture, but illuminated by a full cone. 

 Under these circumstances one would have expected to see a 

 critical image, but not so, and this is the crucial point. In 

 order to obtain the maximum resolving power for that reduced 

 aperture the illuminating cone had to be reduced until three- 

 quarters of the back lens was illuminated," 



It follows, then, that given the three-quarter-cone as the most 

 ideal illuminant, such a cone of an oil immersion objective of 

 r40 N.A. must have a much greater resolving power than the 

 similar cone of a dry objective w^hose maximum aperture is 

 only rO. Up to a certain period dry glasses were the only 

 ones available. All my photographs of the scale were taken 

 under the first conditions ; hence, unless I blundered greatly, 

 they must have made an " advance upon the very earliest 

 methods." 



We now come to the question of oblique light as helping 

 towards the further elucidation of structure in the Podura 

 scale. Mr. Plaskitt says, that when central light is used, 

 "there is insufficient contrast to bring out several of the finer 

 phases of detail. It is much the same as if one placed a 

 piece of plain glass in the midst of a lamp flame and were 

 asked to observe its uneven surface, or to direct a telescope 

 boldly towards the full moon and be expected to trace its finest 

 details." 



The illustrations on the face of them are apt enough, yet, I 

 tear for all that, a little misleading. The conditions of pro- 

 ducing an image in the microscope differ from those concerned 

 with unaided vision upon natural objects. In the microscope 

 it is of a two-fold character ; one geometrically, as in ordinary 

 vision, taking up the centre of the back lens of the objective ; 

 the other, by diffraction, occupying the outer unilluminated 

 ring. Both images reach the eye as one, that produced by 

 diftraction strengthening the image produced in. the ordinary 

 way of vision. Neither with regard to the telescope and the 

 moon do I think the cases are exactly upon all fours with 

 each other. In one, the microscope, the light is tilted to the 

 object ; in the other the telescope remains the same — the moon 

 it is that revolves. Mr. Plaskitt's photographs do not exhibit 

 shadows such as when details in the moon are seen in profile, 

 proving, to my mind, that the conditions of production are not 

 the same. 



Like Mr. Plaskitt I have been unable to discover any 

 dift'erence between the two sides of the scale, though I have 

 had them mounted between two thin cover glasses, to enable 

 me to get at each side. It is, indeed, altogether a puzzle, this 

 secondary structure, and such for the present, I fear, it umst 

 remain. One thing, howe\'er, I would say in conclusion. If 

 my own experience with minute structure is any guide, and 

 with all deference I say this, it is that oblique light will 

 not guide us further, but will only make confusion worse 

 confounded. 



Yours faithfully. 



T. F". SMITH. 



