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KNOWLEDGE & SCIENTIFIC NEWS. 



[June, 1904. 



colour-sensitised plates have been available for many 

 years, and in photo-micrography and other scientific work 

 they have long been considered as indispensable, even 

 those who know the advantages that they offer too often 

 appear to lose their critical sense as soon as they take 

 their cameras out of doors. It is a mistake to suppose 

 that the proper rendering of variously-coloured objects is 

 an artistic matter; it belongs to the realm of science, and 

 the reason that it is neglected is that we have got so used 

 to the conventional errors in the representation of coloured 

 objects in monochrome by photography that they do not 

 offend the eye as errors in outline do. Another reason 

 is, I think, that it has so often been represented of some 

 colour-sensitised plates that they do not need a coloured 

 screen, that many who have seen the results of their use 

 without that assistance have been unable to find any 

 advantage in them. There is no available colour-sensi- 

 tised plate that will give an advantage worth having 

 in ordinary work by daylight, even if detectable by 

 critical examination, unless it is used in conjunction with a 

 coloured screen. The sensitiveness to blue light is so 

 overpoweringly great that the little added sensitiveness 

 to green, yellow, or red is lost unless the blue light is 

 reduced. And the deeper the colour of the screen the 

 more correct will the resulting photograph be so far as 

 the use of well-known commercial screens (or light filters 

 as some call them) is concerned. By the injudicious use 

 of dyes it is easily possible to absorb so much blue that 

 this colour photographs as if it were black, while yellow 

 and green will appear as if white. It is safest not to use 

 a screen that requires the exposure increasing to more 

 than about eight times, unless one has some guarantee 

 that it is suitable. At about this stage, and beyond it, it 

 becomes necessary to adjust the screen to the particular 

 plate that is to be used. It is a mistake to suppose that 

 a coloured screen renders exposures outrageously long. 

 The screen that probably requires a greater increase of 

 exposure than any other on the market, the "Absolutus " 

 screen made by Messrs. Sanger Shepherd, and Co. to 

 suit Cadett's spectrum plates, and it requires exposures 

 out of doors to be increased about forty times, as a rule 

 only needs the giving of a few seconds' exposure instead 

 of a fraction of a second. For hand-camera work, it is 

 well to have a screen that needs the exposure to be 

 doubled, and also one that requires it to be increased to 

 four times or more for use when circumstances permit. 

 It is well worth the little extra trouble involved if only 

 tor the sake of the improvement that will be manifest in 

 the skies, especially when these are partly or wholly 

 cloudless. 



Catatypc. — This interesting process seems to be still in 

 the dcnibtful stage so far as its practical uses are concerned. 

 It was patented nearly three years ago by Messrs. Ostwald 

 and Cirus, and is one of the results of Professor Ostwald's 

 investigations in connection with catalysis. Finely 

 divided metallic platinum or siKer causes the decomposi- 

 tion of hydrogen peroxide when merely brought into con- 

 tact with it. If, therefore, a photograph in which the 

 image consists of metallic silver or platinum is flooded 

 with a solution of peroxide of hydrogen in ether, when 

 the ether has evaporated the peroxide will be decomposed 

 where it is in contact with the finely divided metal, and 

 if the original is a negative there will be on it an invisible 

 positive image in hydrogen peroxide. By pressing such 

 a treated photograph against a gelatine film for about 

 thirty seconds, a notable quantity of the peroxide will be 

 absorbed by the gelatine, and such a " print " can be de- 

 veloped, or made to give a visible result in many ways. 

 An alkaline siher solution will give a black image of 

 metallic silver, an alkaline lead solution a brown image 



of lead peroxide, and so on. By treating such a print 

 with a f( rrous salt, the peroxide will convert the ferrous 

 salt inti a ferric salt and this will render the gelatine 

 insoluble in water. If the gelatine has been mixed with 

 a pigment, as in ordinary carbon tissue, and the print is 

 developed by means of warm water as an ordinary carbon 

 print is developed, it is stated that this method of pro- 

 ducing carbon prints gives the print ready for develop- 

 ment in about two minutes instead of the time usually 

 required to sensitise the tissue with bichromate, dry it, 

 and expose it behind the negative. The process may 

 also be available for photo-mechanical work, for it is 

 stated that gelatine that has absorbed peroxide of hydrogen 

 will take up a fatty ink after the manner of chromated 

 gelatine that has been exposed to light. It is to be hoped 

 that we shall soon hear more of the practical applications 

 of these methods. 



(To he continued.) 



The Antiquity of the 

 Constellations. 



Tl) THE EdITOUS of " KNOWLEDGE." 



Gentlemen, — Let us hope that Mr. Maunder may tell us 

 more about the origin of the constellations. The late Mr. 

 Proctor was very bold and fixed the date at which they were 

 invented (or revealed) at 2170 B.C., neither more nor less. Mr. 

 Robert Brown, too, one would gather, was given (liUe Balbus) 

 to rashness in speculation. 



Mr. Maunder puts the approximate date at 2800 b.c. But 

 some difficulties suggest themselves on the brief summary of 

 his arguments, f./,'. : — 



(i) The centre of the space not included in the ancient 

 constellations must have been the S. pole of the period 

 when they were designed. 



But do we Unow all the ancient constellations ? A recent 

 w-orli, " Sphacra " (referred to below), gives, not 48, but some 

 150. Many are duplicates (and the variants are curious and 

 interesting). Others are quite unidentified, i'.,^'., the market 

 place, the two skulls, tlie stag with two snakes in his nostrils. 



(2) The tradition of the four royal stars marking the 



colures. 

 But Rcgulus was a " royal " star for an obvious astrological 

 reason. It was the heart of the royal beast, the lion, and was 

 supposed to rule the fates of kings. (The star called Cor 

 Hydrae, or the serpent's heart, denotes trouble through 

 women.) If t!ie Persians called other stars "royal" they 

 may have liad ecpially good (or bad) reasons of the kind. 



(3) The date gives the only symmetrical position for the 



actual constellations of the Zodiac. 

 But then- is a strong tradition that they were originally 

 eleven, nut twelve, and their position otherwise is far In mi 

 symmetrical. 



(4 1 The ascending signs at this date laced east ; the 

 descending west. 

 But why did three face nowhere iu particular ? Manilius 

 gives amusing explanations. 



(5) There arc traditions of Taurus leading the Zodiac. 

 Possibly, l)ut the familiar lines of Vergil in the first Georgic 

 do not prove this. 



It is not safe to base arguments on poetry, and, iu fact, 

 Seneca finds fault with the agriculture of this very passage 

 (Ep. 86): "()nr Virgil considered effect more than truth and 

 wished to please his readers, not to teach farming." But the 

 astronomy is right enough. Virgil is thinking of .'\pril, and 

 Ovid's lines (Fast IV. 88) arc the best explanation ; — 

 Nam. quia ver aferil tunc omnia, densaijue cedit 



Frigoris asperitas, fetaque terra patet ; 

 Aprilem memorant ab aperto tempore dictum. 



or 



When that Aprillt with his showrts soote 

 Tlie drought of March had perced to the roote. 



