August, 1902.] 



KNOWLEDGE 



189 



Nvmphon." This maj apparently be considered as just 

 comiriir within the British area of marine zoology, since 

 on the cruise of the " Knight Errant " it was taken at a 

 great dejith to the north of Scotland Imt as far south as 

 latitude 60°. Sir John Murray, of the "Challenger." 

 writing to Dr. Hoek, informed him that the immense num- 

 ber of specimens of this species obtained on that occasion 

 was the largest haul of Pjcuogonida he had ever seen — 

 about a thousand specimens. Sars, when explaining his 

 motive for instituting the genus, observes, " the peculiar 

 circumstance that the young continue to remain firmly 

 adherent to the body of the male, even after they have 

 attained more than half the definite size, is also remarked 

 among the generic characters, as a similar relation, so far as 

 I am aware, has not been found in any other Pycnogonid." 

 Another genus, Paranymphon, with seven jonits instead 

 of five to the second a]>])endages, was founded by 

 Caullery in ISi't" for a species from considerable de])ths 

 in the Bay of Biscay. The same species has since been 

 found also in high northern latitudes. Tjike Boreonijmphon 

 robiistum it is blind. 



The genus Decohpoda, Eights, 1837, requires either an 

 additional family Decolopixlidw, or a widening of the 

 definition of the Nymplmnida}, for its second pair of 

 a]ipendages are ten-jointed. The typical species D. mistraJis 

 was taken in the vicinity of the New South-Shetland 

 Islands. Its chelifori are large with much curved thumb 

 and finger. Of the many specimens obtained all were 

 |>rovided with ovigerous limbs, eleven- jointed, so that 

 Eights counted these in with the four following pairs of 

 legs, and framed a generic name which signifies that in 

 this form there are " ten perfect feet." " The entire 

 animal," he says, " is of a bright scarlet." With this gay 

 picture to contemplate, we may conveniently pause before 

 attempting a survey of the Cryptochclata. 



Conducted by M. I.Cro§s 



PoxD-LiFE Collecting in August. — For the collector of 

 Cyclops, Diaptomns, Waterfleas, and aquatic insect larva?, 

 August is a very capitil month ; not so, however, for the col- 

 lector of the more inte.'esting Infusori i and Rotifera, which 

 are usually quite crowded out by the more vigorous crustaceans 

 in the few remaining ponds and pools not wholly dried up. In 

 larger lakes, however, it is possible to find occasionally a number 

 of interesting forms, particularlj- free-swimming Rotifers such 

 as Asjil'uiilimi //rinilonta and lirightwelli, Sijiu.-hii in pn-liudta and 

 the raier summer forms : Sijinha-ta alylata and r/rnndi.i. Where 

 a "green'' ])ond can be found, full of the flagellate Infusorian 

 EuijleiKt rirklis, there are usually present also a number of 

 RotifiF.-* such as Hi/ddtimi xoita, Knspliura aiiriUi, Diijieiui 

 liiiitphis, etc., feeding on the Euglena. 



In shady forest pools, overgrown with Sphagnum, quite a 

 peculiar fauna of Mos.s-haunting Rotifers will be found, par- 

 ticularly various species of CnUidiiia, Disli/ln. Mflopidin, 

 CathujiiHi, in addition to numerous interesting Rbizopods with 

 shells"of various forms. In similar ponds the large, but very 

 rare. Rotifer ('open.-! sjiicntus should be looked for. Of other 

 Rotifers that may be met with in lakes, more or less abundantly, 

 the following can be mentioned : Brach/onua pain ; Aiiuraa 

 acideaUi, hrechpiiia and hypelanma ; Dinocha'i's pociUunt ; 

 EucMunis ti-iquetni, lii/alhia and oropha ; ilastinocerca hicoriih, 



elonijata and sli/latu. ; Pohjartlira phityptera ; Syiichaeta, treinula 

 and (ihloiign : Pedalion mirum ; and many others. 



ArocHROMATisM .vxi) OTHER TERMS. — So much confusion 

 seems to exist, even at this day, as to wh.at is understood by the 

 term "apochromatism," that it may be well to express, in us 

 intelligible a torn: as possible, the essential qualities which are 

 properly associated with this name, and the difference tluit 

 exists between it and somi-apochromatism and achromatism. 



Apuchromalhm is an exjiression of the highest attainable 

 correction of microscope objectives, comprising the correction 

 of spherical aberration for all colours and the union of three 

 different colours in one focus, that is, elimination of the 

 .stcondarj' spectrum. 



Senii-apijchfomatisia n»y be defined as combining such 

 qualities that the spherical aberration is corrected practiciilly 

 for all colours. It will be understood that in this case the 

 secondary spectrum is present. 



An arhvomatic microscope objective, pure and simple, is one 

 in which, as a rule, defects worse than the secondary sjiectrum 

 are caused by S]iherical aberration of the coloured rays, the 

 spherical aberration being corrected for the brightest ])art of 

 the spectrum only. Generally in this correction two different 

 colours only are united in the same focal point. 



In order that a clear conception may be formed of the above 

 remarks, it will be well to append a brief explanation of the 

 Serutidary Sj)eclram. In an achromatic lens the clirom.atic 

 aberration is corrected for the brightest (yellow and green) rays 

 of the spectrum, and the pronounced colour shown by un- 

 corrected lenses is, in consequence, removed. 



A stricter examination, however, shows that raj's of a different 

 colour are not brought to the same focus, for, owing to the fact 

 that flint glass, as compared with crown glass, disperses the more 

 refrangible rays relatively too much, and the least refrangible 

 relativelj" too little, a peculi.ir secondury spectrum results from 

 the achromatic combination, the rays corresponding to the 

 brightest apple-green part of the ordinary spectrum being very 

 closely united and focussed nearest the combination, whilst the 

 other colours focus at increasing distances hi j)a>ni, yellow being 

 united with dark green, orange with blue, red with indigo. The 

 composite effect of these colours is best seen with oblique Ught, 

 causing dark objects to have apple-green borders on one side 

 and purple ones on the other. 



Prepar.\tiox of Metal Specimens for the Microscope. — 

 In Knowledge for November, I'.Hll, reference was made to the 

 microscopic examination of metals in general terms, and as some 

 considerable interest was evinced in the subject, the following 

 notes have been prepared to guide those who may wish to 

 experiment for themselves in the preparation of metal specimens 

 for examination. 



The preparation of specimens of metal for the microscope 

 involves the greatest care ; the principal object being to obtain 

 a perfectly level surface, free from all scratches and marks, with 

 the highest degree of polish. This will be better illustrated by 

 an example. 



The student h.aving obtained a sample of metal, the first thing 

 to do is to carefully file or grind the surfaces he wishes to 

 examine. The marks thus made must be taken out with a very 

 smooth file or emery cloth, gradually diminishing the coarseness 

 of the cloth until he reaches the finest grade of all. 



From this stage the polishing must be done on parchment or 

 chamois leather stretched very tightly on wood : the leather 

 being covered with fine crocus powder or rouge moistened with 

 a little water. 



This is the most important stage of the specimen, especially 

 it the metal be very soft, and the student should freiiuently 

 examine the metal through the micro.<cope— a matter of a few 

 moments only — by clamping it in one of the new metal holders 

 which have been recentlj' introduced by ^lessrs. R. & J. Beck 

 and Watson A Souf. The.se holders are fitted with jaws in 

 which the subject is firmly gripped, and provide for setting in 

 any plane, and obviate the necessity for mounting the specimens 

 on glass slips. 



Oq examination it will be seen that parts stand in very high 

 relief. This is the object of the leather polishing — to gradually 

 grind away the soft and leave the hard parts, and great care 

 should be exercised. 



Having obtained the requisite degree of polish the specimen 



