66 



NATURE 



{_May 1 6, 1878 



used, accomplishes all that is required. Hence the fine "new 

 formula " lenses, dry (also provided with fronts to be used as 

 immersion lenses), are as yet an unsurpassed boon for this special 

 class of work. And certainly it is one which, in relation to 

 biology, has a most important future. I know of course, that 

 the optician has irresistible limitations to deal with ; but the 

 " new formula" dry lenses I have referred to, prove, in comparison 

 with the preceding lenses, made by the same firm, that the dry lens 

 was capable of most serviceable improvement. The same applies 

 to a ^th-inch lens, made recently at my request by the same skilful 

 makers. As an analytical optical instrument, it is possessed of 

 capacities far greater than are represented by its mere increase 

 of magnifying power over the -j^th-inch objective, by the same 

 makers ; and equally so in relation to their ^jjths of six or seven 

 years ago, when the superior magnifying power of the latter is 

 considered. And yet the ^^th-inch and the -^th-inch to which 

 I refer, were admirable glasses, and have done excellent service. 

 What is important, therefore, is that the larger demand for 

 lenses that will "resolve" readily, difficult lined and beaded 

 objects, which can certainly be best done, all things being equal, 

 with "immersion" lenses; and to the improved manufacture 

 of which Carl Zeiss' oil immersion gives apparently a new 

 departure : should not lead the best opticians in England, the 

 Continent, and America to abandon efforts for the still greater 

 improvement of their dry lenses. They are of the greatest value to 

 the practical biologist, working amidst the minutest living things 

 in Nature, and from the study of which so much may be antici- 

 pated. 



There is another feature in the use of this lens which is a 

 drawback. The essential oil is a solvent of most of the 

 varnishes and gums used in mounting, and "finishing," micro- 

 scopical "slides;" and consequently some of our cherished 

 "tests" — placed near the edge of the cover, and which we have 

 been in the habit of using for years, will not serve us. And 

 this, of coiu-se, has a wider application. But this may be over- 

 come by coating the edge with shellac-varnish, which the oil does 

 not dissolve ; only this is extremely brittle, and is not to be 

 depended on. 



But it is further necessary, in using this len«, that the objects 

 should be mounted in balsam, or some other fluid with an equal 

 refractive index. The majority of "dry" mounted objects are 

 by no means better shown by this lens than by an ordinary im- 

 mersion lens. But this may be overcome if the objects, such as 

 f rustules of diatoms, be "burnt" on to the cover. This inti- 

 mately unites the crown glass cover and the object, making 

 them practically one. If this be not done the ray coming from 

 the object has to enter air before passing into the lens, so neu- 

 tralising the special properties of the glass. But here again the 

 j/^na/ objects — used, for example, as "tests"— and obtained as 

 the result of years of careful selection, are of no avail. 



But this glass will be of great value in the study of rock 

 structures, &c., because the oil will render them transparent 

 without special polishing ; and its great working distance will 

 in such work be a great boon. 



It may perhaps be right to note that this lens, although not 

 provided with the complex arrangement of " screw -collar 

 adjustment," and although only " immersion," is higher in price 

 than the most costly ^^th by any English maker, although the 

 latter lens may have the screw collar correction, and be both 

 " immersion" and dry, W. H. Dallinger 



St. James's Parsonage, Woolton, Liverpool, May i 



Science for Artists 



In Nature, vol. xviii. p. 29, there is an article upon 

 "Physical Science for Artists," in which one of my pictures is 

 thus described: "No. 309. The Sunrise Gun, Castle Cornet, 

 Guernsey— Tristram Ellis. Sky colour good ; impossible colour 

 of water under sky conditions given." 



It is not usual for an artist to answer a criticism, but in this 

 instance I do so purely upon scientific grounds. The water 

 shown is slightly ruffled with a breeze blowing towards the spec- 

 tator, and hence reflects a part of the sky which makes a greater 

 jangle above the horizon than the reflection makes below it. The 

 central part of the sea would reflect that portion of the sky which 

 is at the very top of the picture, and if the critic will kindly re- 

 examine, he will find the colours of those parts almost identical. 

 As the sky gets greener towards the zenith with the given kind 

 of sunrise, the sea appears greener than the portion of the sky 

 shown, and this effect is heightened by the strong gi-'!en local 



colour of the water in the shadows. The sea was painted after 

 careful consideration and study direct from nature, and remem- 

 bering the breeze is nearly parallel with the line of vision, is, I 

 think, correct. If the wind had been at right angles to this line 

 the colour would have been quite different, and perhaps this is a 

 matter which the writer of the article did not at the moment take 

 into consideration. Tristram Ellis 



Kensington, May 10 



Time and Longitude 



There is a practical answer to the problem put by Mr. 

 Latimer Clark (Nature, vol. xviii. p. 40^ As a matter of 

 fact the day begins, or rather the day is first named at the 180' 

 meridian east or west from Greenwich ; but this initial line, if I 

 may call it so, diverges in the South Pacific to about 170° west 

 from Greenwich, bringing many of the islands, as Fiji, Friendly, 

 Sunday, Chatham, &c., into the same date with the nearest 

 civilisation, Australia and New Zealand, Asia, &c. Without 

 notes I cannot trace this line acciu-ately between the Isles, but to 

 take certain cases. Fiji counts its day east from Greenwich, 

 Hawaii and Society west from Greenwich. At this moment I 

 forget which division the Navigators enter, so to answer the 

 problem. Where did last Monday begin? — At about 170" 

 west longitude. Where did it end? — At 180* west in North 

 Pacific, How long did it exist ? — At any one place twenty-four 

 hours, but taking adjacent places on either side of the initial line, 

 Monday will have been a date during forty-eight hours ; or if 

 a vessel should be just on the eastern side of the 180° meridian, 

 and keeping, as she should, Greenwich time through Ame- 

 rican route, Monday will have been a date during very nearly 

 forty-nine hours. 



The case proposed by Mr, Latimer Clark is no hypothetical 

 one. During the war of 1855 the squadron in the Pacific was 

 sent across to co-operate with the fleet in China. It found itself 

 a day behind the China fleet as it had entered the Pacific round 

 Cape Horn, whilst the China fleet had passed round the Cape of 

 Good Hope, and for a short time the two fleets side by side kept 

 different days. Again the steamers from San Francisco to Japan 

 alter their dates temporarily whilst in Japan to suit the local 

 reckoning, and enter both dates in the log. J. P. Maclear 

 May 13 



Menziesia Cserulea 



I AM rather surprised to see it stated by the Rev. 

 M, J. Berkeley in Nature (vol. xviii. p. 15) that the late 

 " Dr. Thomas Thomson was so fortunate, after three times 

 ascending the Sow of Atholl, as to rediscover the long 

 lost Menziesia ccerulea." I doubt if it was ever lost, cer- 

 tainly it has not been long lost. I find, on looking over my 

 Herbarium, that my specimen was collected August 6, 1867 ; 

 since then I have heard of it having been found by others. I 

 saw several plants which I left, and I have little doubt that some 

 of them are there still. Fortunately the preservation of the 

 plant is due to the following circumstances : — 1st. That it 

 flowers in May ; few botanists visit the Highlands till later in the 

 year. 2nd. The plant has a considerable general resemblance 

 to Empetrum nigrum. I have seen them growing in the same 

 tuft ; in such a case it requires a very sharp eye to distinguish 

 one from the other even at a short distance. 3. The plants are 

 widely scattered over the hill, so that it would require days to 

 enable any one to say that it was lost ; indeed no plant is likely 

 to be lost so long as the natural conditions remain unchanged. It 

 may be stolen but not lost. I take for granted, of coiurse, that 

 every true botanist will be merciful in such a case. 



Edinburgh, May 6 Alex, Craig Christie 



"Hermetically Sealed" 



What is hermetic sealing ? I have been under the belief that 

 it means sealing with the material composing the object to be 

 sealed ; as in the case of sealing a glass tube in the spirit-lamp, 

 M. Bordier's charming paper on the Greenland Eskimo (Nature, 

 vol, xviii, p, 16), says that an aperture in a hut is hermetically 

 sealed with goldbeater's gkin ; and that a fisherman is hermeti- 

 cally enveloped round the loins by a leathern bag. You may, 

 perhaps, think it worth while, in the interest of accurate 

 scientific terminology, to settle the point. 



May 10 



W, T, 



