146 



KNOWLEDGE 



May, 1915 



Since no reaction takes place in any of the tubes 

 which contain as much as twenty per cent, of the 

 dominant white extract, and since the tubes con- 

 taining the recessive white extract are all oxidised, 



it is clear that the dominant whites are white. 

 because they contain an inhibitor or anti-tyrosinase, 

 which prevents the formation of pigment ; and 

 that the recessive whites are white, because at least 

 one factor necessary for the formation of the 

 pigment is absent, namely, tyrosinase. It was 

 also found possible to isolate this inhibitor by 

 fully saturating the fluid extract with ammonium 

 sulphate, when the inhibitor is completely pre- 

 cipitated. 



This account of the chemical cause of the differ- 

 ence between dominant and recessive whiteness 

 agrees also with the genetic data ; for, as has 

 already been said, $ dominant white (with inhibitor) 

 X ? self-colour (with colour-producing factors) gives 

 nothing but white offspring; but <? recessive white 

 (without colour - producing factors) x ? self-colour 

 (with colour-producing factors) gives nothing but 

 self-coloured offspring, as would be expected. 



The fact that dominant white animals sometimes 

 have coloured spots or patterns must be explained 

 by the supposition that in these places the inhibitor, 

 for some specific reason, has not been deposited 

 in the skin. No doubt recessive whites are some- 

 times piebald, for analogous reasons. 



The hypothesis that dominant whiteness is 

 caused by the presence of a chemical inhibitor 

 in the cells of the skin, which can be transmitted to 

 the offspring, and which prevents the formation of 

 pigment, is by no means a new one. It was first 

 discovered that certain dihydroxyphenols, such as 



orcin, had the property of inhibiting the action 

 of tyrosinase upon tyrosine in vitro. The dominant 

 white flowers of Primula sinensis also contain an 

 inhibitor, which can be destroyed by hydrogen 

 cyanide, after which they will show certain oxydase 

 reactions with suitable reagents. Certain pale 

 shades of flowers which are dominant over the 

 deeper shades have also been thought to be due to 

 the presence in the petals of deoxidising substances, 

 such as tannin or sugar ; and, lastly, the colour of 

 the larvae of Spelerpes bilineatus has been modified 

 by subjecting the eggs to a dilute solution of orcin. 



Figure 125. — A series of tubes containing the skin 

 extracts, and so on, after twelve hours' in- 

 cubation. 



A. — Extract from black rabbits -f- tyrosine -f-H 2 2 . 



B. — Extract from black rabbits -4- tyrosine -4- H 2 2 

 at room temperature. 



C. — Extract from black rabbits -+- tyrosine -4-H a 2 

 + extract from dominant white rabbits. 



D. — Extract from chocolate rabbits + tyrosine 

 + H. 2 2 . 



E. — Extract from black rabbits+/>-cresol. 



F. — Extract from recessive white rabbits -(-tyro- 

 sine -|-H 2 0.,. 



C shows no sign of oxidation, because it is in- 

 hibited by the anti-tyrosinase in the dominant 

 white rabbits, and F shows no oxidation, because 

 recessive white rabbits contain no tyrosinase. 

 E is coloured deep pink. 



Note the layer of unoxidised fluid at the bottom of 

 D, where it has not been in contact with the atmo- 

 sphere. This layer is not so clearly defined in the 

 other tubes, as they were slightly shaken. 



NOTES. 



ASTRONOMY. 



By A. C. D. Crommelin, B.A., D.Sc. 



F.R.A.S. 



MELLISH'S COMET has been observed several times. 

 It has a sharp nucleus, resembling a star, and a considerable 

 coma surrounding it, the diameter being about 10'. Mr. 

 W. H. Steavenson traced the tail for half a degree on April 

 7th. The orbit given last month needs sensible correction. 

 Perihelion passage will be about July 17-63 G.M.T., and 

 the logarithm of the perihelion distance is 0-0068. 



The following outline ephemeris gives an idea of its 

 motion from June to October :■ — 



Date. R.A. S.Dec. Log. r. Log.A. 



1915. h. m. s. o , 



June 17-63 ... 4 5 16 ... 67 28 ... 00552 ... 9-6655 

 July 17-63 ... 6 6 56 ... 39 37 ... 0-0068 ... 9-9326 

 Aug. 16-63 ... 6 26 16 ... 28 21 ... 00552 ... 0-0568 

 Sept. 15-63 ... 6 25 40 ... 21 50 ... 0-1518 ... 00860 

 Oct. 15-63 ... 5 52 52 ... 16 9 ... 0-2466 ... 0-0706 



The path is more favourable for northern observers than 

 that given last month. Southerners will enjoy the best of 

 the display, but the comet returns to our zone of visibility 

 in September, when it may still be a naked-eye object. 



Winnecke's Periodic Comet returns to perihelion at the 

 end of August, and was detected early in April at the Berge- 

 dorf Observatory, its magnitude being 16. 



STELLAR SPECTRA AND TEMPERATURES.— 

 Messrs. W. S. Adams and A. Kohlschiitter write on this 

 subject in No. 89 of the Mount Wilson Contributions. It 

 is taken for granted that the stars of large Proper Motion 

 are on the average nearer than those of small Proper 

 Motion. The latter have the violet end of the spectrum 

 weaker than the former (for the same spectral type) ; also 

 the hydrogen and a few other lines are relatively stronger 

 in the small Proper Motion stars. The conclusion is that 

 these spectral differences do not arise from absorption 

 in space, but arise from greater atmospheric absorption in 

 stars of large mass. The small Proper Motion stars, being 

 more distant than the others, are presumably of greater 

 mass. Hence there is hope of deducing the absolute mass 

 and luminosity of stars by careful study of the intensity of 

 certain lines, and also the relative strength of different 

 parts of the continuous spectrum. The results have to be 

 considered in combination with the spectral class to which 

 the star belongs. 



LATE SPECTRA OF NOVAE.— No. 87 of the same 

 Contributions gives reproductions of photographs of the 

 spectra of 4 Novae taken with the sixty-inch at Mount 

 Wilson by Messrs. W. S. Adams and F. G. Pease. They are : 

 Nova Aurigae, twenty-three years after outburst ; Nova 

 Persei, t irteen years after; Nova Lacertae, four years after ; 



