April. 1911. 



KNOWLEDGE. 



143 



;uid the high tides in other localized places upon the special 

 configuration of the coast — the narrowing of the waterway, 

 as also in the Severn and the Wye in England. 



If the moon raises tides in the ocean, a much higher tide 

 should she raise in the lighter air ; if so, does her attraction 

 counteract the effect of the increased weight of air on the 

 barometer ? 



.'^t the commencement of his letter Mr. Strickland says: "I 

 hold that the negative they (the official meteorologists) think 

 they have proved, is not proven." (Do not their statements 

 rest on facts observed at Greenwich ?) 



In your September number, — page 372 — Mr. Ditcham writes 

 to show that the high tides in the Fraser River bring up 

 warmer water from the sea, and so warm the climate, but is 

 not that almost as indirect a result as it would be if they 

 brought up a log of wood which 

 he cut into firewood and warmed 

 his house with ? Or again, if 

 the liver were dammed back, 

 the warming effect of the moon 

 would be prevented to those 

 above the dam. I li\e on the 

 watershed between the Thames 

 and the Channel where there 

 are no tidal streams, and can 

 find no sign of the moon"s in- 

 fluence on our weather. 



There is also a letter from 

 Mr. Ditcham. in your February 

 number of this year, the figures 

 of which, I am sorry to sa\', I 

 cannot understand, whether they 

 give the height of tides, or some 

 barometric weights, I do not 

 know, nor can I see any con- 

 nection between the figures and 

 the dates. L. J. 



rODUK.A. SC.\LES. 



To the 

 Editors of " K.xowi.EDGE." 



Sirs, — Mr. Plaskitt. in his 

 courteous reply to my letter, 

 has almost entirely ignored the 

 point I tried to drive home, that 

 the available aperture of any 

 wide apertured micro-objective 

 is limited to the back lens 

 being three-quarters filled with 

 white light only. That I filled 

 up the whole of mine with such 

 light, 1 ne\er asserted. Indeed. 

 1 could not have done so had I 

 tried, my condenser being a dry 



one. For all that, I could always break down the iin.age in my 

 -objective, of 1-40, upon any object mounted in balsam, by 

 using the largest stop of the condenser. What I did claim, 

 howe%er, and do claim now, was that I worked the objecti\e 

 with the largest aperture it would stand — rather a different 

 matter. 



Upon the points he has raised, save one, 1 have nothing to 

 fight ; the laws of refraction are fixed. It is only upon the 

 application we differ. Neither is it necessary on my part to 

 try the experiments he suggests in the last paragraphs of his 

 letter, for already I agree. 



This question of full versus available aperture is a very old 

 one with me — twenty years old, in fact. On referring to the 

 back numbers of the journal of the Ouekett Club, I find that 

 Mr. Ingpen raised exactly the same point in 1S91, in connection 

 with some objects exhibited by myself. He said that he 

 " wanted to employ the greatest powers of the objective to 

 be obtained between 1 and 1 • 4, and did not see how they 

 could be made use of upon a dry object,"' — at the same time 

 advocating the use of a dense medium. 



To this, Mr. E. M. Nelson replied that he " found, on the 



"Ml^! 



Figure 2. 



Part of a Podura Scale, showing a portion of the right hand 



side of the Scale reproduced in " Knowledge," Volume 



XXXIll. p.age 535, Figure 2, enlarged 2i times to show 



intermediate lines. 



other hand, if the object was drj- on the cover glass, it would 

 bear the test better than if in a mediur;i. So long as it was in 

 optical contact with the front of the lens they could get in all the 



spectra Photographs of objects mounted in the 



denser medium looked all smeared over." He had never seen 

 a decent critical im.age produced from anything in a dense 

 medium. He did not know the reason, unless it might be 

 that the treatment undergone by such might have the effect of 

 spoiling them. 



This appears to be a question of theory versus practice, and 

 reminds me of the German Professor who forbade his students 

 to adopt a certain formula, because, though it worked out well 

 enough in practice, in theory it was all wrong. In the present 

 instance the theory was that an oil-immersion objective must 

 have an oil-inmiersion condenser of the same aperture, to 



develop it. Workers bought the 

 oil-immersion condensers and 

 then innocently stopped them 

 down until they got the working 

 image, thinking all the while 

 that they were utilising the full 

 aperture of the lens. My chal- 

 lenge to them was to produce 

 something under an oil-innner- 

 sicm condenser I could not show- 

 equally well with a dry one ; a 

 challenge ne\er taken up. This, 

 howe\'er. is \'ery old historj' now. 

 I shall be only too pleased 

 for the Editors to send Mr. 

 Plaskitt my address, and equally 

 pleased to receive the enlarged 

 prints promised. I know how- 

 easy it is to miss little points in 

 a small print which are perfectly 

 obvious when they are further 

 magnified. May I suggest, liow- 

 e\er, that with the Editors' 

 permission, Mr. Plaskitt sends 

 one enlargement to be repro- 

 duced for the benefit of the 

 readers of " Knowledge." I 

 am sending one with this letter 

 hoping for that permission, 

 because, I take it. the truth is 

 what we both want. If this be 

 granted, that is, the Editors' 

 consent, the one of his to go 

 best with mine would be the 

 one with the oblique lighting 

 across the scale. 



Yet, even then, the credi- 

 bility of appearances will still 

 remain an outstanding question. 

 I. at least, am not prepared 

 to state with certainty what the secondary lines denote, and I 

 do not suppose Mr. Plaskitt is more positive. We can only 

 judge of the relative truth by the method of production. 

 Leaving my own opinion out of the question altogether, the 

 weighty authority of Mr. E. M. Nelson and the late Dr. 

 Dallinger cannot be ignored, who both advocate the central 

 cone of illumination, as opposed to oblique light. The last, in 

 his presidential address to the Ouekett Club, in 1891, 

 speaking of the new apochromatism, says : '' It gives certainty 

 and precision to all work done .... but w-e must 

 be careful not to re-introduce the ghostly element by false 

 interpretation. I am increasingly convinced of the possible 

 danger of employing shafts of oblique light only in one azimuth. 

 The peril of misinterpret.ation is enormous." 



Again, of the new apochromatic of 1-60 N..-\., it is claimed 

 that '■ it is a triumph of the optical firm w-hich produced it. 

 . . . . But I would hasten to say, that I would not 

 trust a single result produced by its means, when oblique 



light in one azimuth is employed It is fatal to its 



truth. We can absolutely .get almost any desired result with 

 it. It is a very optical Witch of Endor for calling up ghosts 



