NOVKMUIiU. 1911. 



KNOWLEDGE. 



425 



by any means uncommon amongst the Ligurian .Alps, but I 

 have never observed them except immediately after a thunder- 

 storm, and they are apparently developed in that part of the 

 thunder-cloud which represents the pointed end of the 



Thor's .Anvil," as it is sometimes called, — the long smudge 

 which streams out behind at the opposite extremity to tlie 

 advancing cauliflower-liUe front of the cumulus. 



On the occasion on which the two sketches [Figures 2 and 

 i] were made, the whole sky was covered with these clouds. 

 I took a photograph of part of it. but the result was too uniform 

 in tone to come out well in reproduction. In my sketches I 

 have slightly exaggei'ated the light and shade, but the forms 

 were drawn with all possible care and accuracy. The rounded 

 out lines of the pendant clouds appear to be due to the fact 

 that the clouds are descending — that they are. in fact, cumuli 

 upside down. After one of these storms we had the rather 

 rare pleasure of seeing a perfect and very beautiful lunar 

 rainbow (July 11th). ^^,^,^ PARKINSON (M.A.. O.xon.) 



OMPOSITIOX OI- T.ACHINIDAE. 

 To the Editors of " Kxovvledgh." 



Sirs, — I observed two of these flies the other day (July 25th, 

 1911) while attempting to lay eggs on the larvae of GlottiiUi 

 doniinica, which were feeding on the leaves (and inside 

 them, under the epidermis! of a species of Crinum. 



One of them was feeling a larva near the head with its 

 front pair of legs, as it now and again extended its ovipositor 

 forwards under the thorax (I saw no eggs actually deposited). 



The other saw the movements of a larva which was feeding 

 under the transparent epidermis while about two inches off, 

 and rapidly making its way there apparently succeeded in 

 laying its eggs through the epidermis of the leaf on to the 

 caterpillar, but of this I cannot be certain. The former larva 

 was crawling on a neighbouring leaf and was not under cover, 

 but it defended itself by swinging its tail round and 

 discharging some liquid at the offending irritation. 



I have noticed that a Tachinid is usually recognisable by 

 the way in which the antennae stick out from the head during 

 life, but the antemiae of these two flies seemed to be in danger 

 of getting broken off, and they reminded me irresistibl\' of a 

 terrier's tail when he smells a rat ! 



L. Ct. GILPIN-HKOWX. 



STANUAKL) TIMi: IN ToKTUd LKSE TKKRITORIES. 

 To the Editors of " Knowledge." 



Sirs, — I beg to inform you that Standard Time will be in 

 use from 1912, January 1st, throughout Portuguese territories, 

 as follows : — 



S*" 0"' E. Macao, Portuguese Timor. 



5" 0" E. Portuguese India (provisionallv 5" 30'" E.I. 



2" 0" E. Portuguese East Africa. 



jh gm £_ Portuguese West Africa. 



O'' 0" (Greenwich, or West Europe). — Portugal, St. 



Thome and Principe Islands, Whydah. 



l'' 0™ W. Madeira, Portuguese Guinea. 



2" 0" \V. Acores, and Cape Verde Islands. 



This Observatory remains entrusted with the determination 

 and the telegraphic transmission of Standard Time to the 

 whole country, to the Lisbon Time-ball, and to the Time 

 Stations at the Meteorological Observatory, Ponta Delgada 

 (St. Miguel, Acores). 



I take this opportunity to state also that the most reliable 

 geographical latitude of this Observatory is : 

 Lat. N. 38° 42' 30"-5 (prime vertical, meridian, and zenith 

 telescope series of observations 

 from 1872 to the present, printed 

 or unprinted), 

 and that the name " Lisbon, Tapada " is now the most 

 suitable for it, like, for instance. " b'lorcnce, Arcetri," or 

 '■ Naples, Capodimonte." 



There has been built, and has now been working for two 



years a new astronomical observatory at Lourencjo Marques, 



whose geographical coordinates are (transit pier) : 



Lat. S. 25°58' 4"-9 + ()"-2 (Meridian observations by 



Capt. Gago Coutinho.) 

 Long. E. 32° 35' 39" -4 + 0"-05 (Moon culminations, simulta- 

 neously here, and geodetic 

 connections with the Cape. ) 

 Altitude (top of pier) 59 metres. 



CANYOS RODRKjL'ES, 



Vice-Admiral, P.N., Director. 

 Observatorio .Astronomico de Lisboa. 



DARK-GROUND ILLUMINATION AND ULTRA- 

 MICROSCOPIC VISION. 

 To the Editors of " Kn'owi.euge." 

 Sirs. — In view of the great number and variety of dark- 

 ground illuminators and appliances for ultra-microscopic 

 observation which have come into existence, it is a matter for 

 regret that makers ha\e not always been felicitous in the 

 choice of descriptive terms, and it is not improbable that in 

 many cases this may have encouraged a pretty widely spread 

 tendency on the part of users of the microscope to confound 

 enhanced visibility with increased resoKing power, witli the 

 result that a considerable amount of cc^nfusion has arisen 

 respecting the fundamental aspect of the two modes of 

 observation. 



Neither the method of dark -ground illumination nor the so- 

 called ultra-microscope can in any true sense of the term be 

 regarded as a means of enhancing the resolving power of the 

 microscope. In accordance with the undulatory theory of 

 light this can only be accomplished by increasing the aperture 

 of the optical system or by diminishing the wave length of the 

 light. In their fundamental physical aspects the method of 

 dark-ground illumination and the obser\ation with the ultra- 

 microscope are identical, and both ser\e to enhance the 

 \ isibility of an object. 



.At the bottom of the secret which underlies these methods 

 is the simple and familiar fact that brightly illuminated objects 

 can be seen more distinctly on a dark back-ground than on 

 one which is itself bright. Two things happen when a bright 

 object is seen on a black ground ; objects which were visible 

 on a bright ground become much more distinct, and other 

 particles which could not be seen before will come into view. 



When this principle is applied to the microscope it is found 

 that in the field furnished by the method of dark-ground 

 illumination details can be observed, especially in preparations 

 containing micro-organisms, which cannot be recognised under 

 ordinary circumstances, though dimensionally they are well 

 within the resolving power of the microscope. In addition, 

 particles become so far visible that their presence can be 

 percei\ed, and this despite the fact that their dimensions may 

 be considerably smaller than the wave length by which thev 

 are seen, and accordingly beyond the resolving power of the 

 microscope. 



So long as the objects as seen in a dark field exhibit structural 

 details and well defined contours we are dealing with simple 

 dark-ground illumination. When, on the other hand, the field 

 is seen to contain particles in which there is not a visible trace 

 of detail and which accordingly present the appearance of 

 bright point-like discs, generally surrounded by bright and 

 dark rings, the case is one of ultra-microscopic observation, 

 and the particles whose presence is thus perceived are ultra- 

 microscopic. 



It will thus be seen that the dift'erence lies solely in the 

 manner of observation and not in the nature of the apparatus. 

 On the other hand, ultra- microscopic observation implies 

 dark-ground illumination, whilst it is not every form of dark- 

 ground illumination that constitutes an ultra-microscope. For 

 this reason it is most desirable that the various appliances 

 should bear appellations from which it is at once apparent 

 that they are primarily intended for dark-ground illumination, 

 pure and simple, or for ultra-microscopy, as the ease mav be, 

 i.e., dark-ground illuminators should be named so as to 

 distinguish them from ultra-microscopes. 



It should, howc\er, not be oxerlooked that in practice it 



