404 



KNOWLEDGE & SCIENTIFIC NEWS. 



[April, 1906. 



deeply-coloured patches throughout the majority of 

 species of mammals and birds, sometimes as large and 

 conspicuous patches, e.g., hindquarters of the rabbit, 

 rump of bullfinch, and at other times only distinguished 

 from the surroundmg parts by differences of shade so 

 slight as to be incapable of serving any warning, pro- 

 tective, or other similar purpose. 



It is then pointed out that in some cases the " poeci- 

 lomeres " are only visible as transitory patches during 

 the time an animal is undergoing a change of colour. 



The head of the stoat and of a young shoveller drake 

 are cited amongst others as examples of the change 

 taking place along precisely similar lines, whence it is 

 argued that as the outward effect is so slight and transi- 

 tory, and as the process exists in two ;uiimals so widely 

 separated, the fundamental cause must be a deep- 

 seated physiological one. 



We, therefore, claim to have shown that where con- 

 ditions for high vigour exist the majority of the animals 

 will be brightly coloured, and suggestions are made to 

 account for the apparent exceptions to the rule, wliich 

 suggestions aie borne out by the distribution of the 

 dark I'olar and light tropical species. 



We have also shown that shortly before a moult, 

 in many animals, the colour of the pelage fade.s, 

 beginning along certain definite areas, and from certain 

 centres which I have called " poecilomeres," and that 

 this bleaching is due to physiological causes. 



Further, that " poecilomeres " may be found 

 throughout the mammalian and avian series, as patches 

 differing either in their intensity or lack of colour from 

 the surrounding portions, in many cases conspicuous, 

 and cited as examples of warning, protection, Sec, or, 

 again, as marks so slight as to be unnoticeable unless 

 carefully looked for, or yet again, merely appearing as 

 transitory patches during the growth of a new pelage 

 or plumage. Hence it is argued that these patches, so 

 universal, and in many cases so inconspicuous, must 

 owe their inception to internal rather than external 

 causes, and that we have here the basis of diversity of 

 colour in coloration. If these facts then be accepted, 

 both colour and coloration must be due to physio- 

 logical causes. 



The question of whether seasonal changes are pro- 

 duced by moult or by colour has purposely been en- 

 tirely omitted as belonging to a side issue. 



Natural selection and protective coloration have 

 also been left out, not because we do not believe in the 

 great part they play, but because, if our suggestion be 

 near the truth, they are only able tO' make use of those 

 colours or modify those markings which in the first 

 place are supplied by " vigour." 



Tuition by Correspondence. 



The University Examination Postal Institution of 27, South- 

 ampton Street, Strand, publishes a series of handbooks 

 which are specially and excellently adapted for Cambridge 

 Higher Local Candidates. The series includes a guide to 

 this examination, giving full details as to method of pre- 

 paration, and advice as to text books, together with reprints 

 of papers set at previous examinations. .As each paper is 

 printed en a separate perforated sheet, it can be used by 

 schools for examination purposes. The papers are kept up- 

 to-date by the insertion once a year of the latest papers. To 

 the French and arithmetic papers full answers by F. 

 Thomas. B..\., B.Sc, and W. H. Dines, B.A., F.R.S., are 

 published in addition to the questions. Both the method and 

 the subject matter of this valuable little series of handbooks 

 are well arranged, and we can confidently recommend thein 

 to the perusal of candidates and students. 



PhotogrsLpKy. 



Pure and Applied. 



By Ch.^pman Jones, F.I.C, F.C.S., &€. 



Uncorrected Lenses. — A week or two ago, the Fhoto- 

 Club, of Paris, opened, in their rooms, an exhibition of 

 photographs made by means of lenses uncorrected or 

 only partially corrected for chromatic aberration, and 

 it appears that in due time a collection of these prints 

 will he shown in London by the Royal Photographic 

 Society. Uncorrected lenses have been used, and used 

 purposely on account of the character of the image that 

 they give, for long enough, but the lenses that this 

 collection illustrates the use .of, have been recently de- 

 signed by MM. L. de Pulligny, and C. Puyo, and are 

 issued commercially by L. Turillon, of Paris, under the 

 name of " Anachromatic " lenses. Thev may be em- 

 ployed in the form of single lenses, symmetrical 

 doublets, or with a negative lens behind a positive, as in 

 lelephotographic combinations. So far as can be 

 gathered, it appears that the effects of chromatic aber- 

 ration are specifically valued, as giving- what a perfect 

 lens would render as a sharp line, in the form of a 

 diffused line that is darkest down its centre and 

 gradually becomes lighter as the distance from the core 

 increases. In this way, detail is softened and fine 

 detail is averaged down to a comparatively level tone. 

 These desiderata will be appreciated by many artists 

 who photograph only for the purpose of making 

 " pictures." 



Of course, such lenses must be of greater focal 

 length for a given plate than the modern anastig- 

 mat or even its predecessors need to be, because of 

 iheir inferior covering power. The aim of the optician 

 is to improve definition and increase covering power, 

 and quite rightly so, but if the pictorial worker does not 

 want these improvements, he is a positive loser in hav- 

 ing instruments that possess them. It is the photo- 

 grapher's ignorance of optical matters that leads him 

 to imagine that the more costly a lens, the better it 

 should serve his purpose. Whether it will do so or 

 not depends entirely on what his purpose is, and wherein 

 the special value of the lens consists. The greater 

 focal length is generally a distinct advantage in pictorial 

 work, and granting this, the need for costly lens 

 systems does not exist, even when fully corrected lenses 

 are required. Complete descriptions of the " .\nachro- 

 matic " lenses are not to hand, and therefore the novelty 

 of their construction, if they are in any way novel, can- 

 not be now given. 



Improvements in " Cooke ^' Lenses. — Mr. H. Dennis 

 Taylor, of York, the inventor of the " Cooke " lenses 

 that are so well-known to photographers, is still seek- 

 ing to improve them and increase their usefulness. 

 The three single lenses that constitute the ordinary 

 Cooke objective cannot be separated into two complete 

 parts of greater focal length like so many modern 

 lenses that thus give in one instrument the choice of 

 two, or, if not symmetrical, three focal lengths. Mr. 

 Taylor has devised a means of obtaining this advantage 

 bv duplicating the usual system of three lenses, making 



