June 17, 1897] 



NATURE 



153 



LETTERS TO 'THE EDITOR. 

 [The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond tvith the writers of, rejected 

 Manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications. '\ 



On a Method of reproducing Astronomical 

 Photographs. 



Prok. Max Woi.k, in his excellent article on the " Reflector 

 and Portrait Lens in Celestial Photography," published in your 

 issue of April 22 last, mentions a method of "reproducing 

 nebulous masses " from original negatives, and refers to my re- 

 production of a photograph of the nebula M. 8, done in collabora- 

 tion with Mr. Lunt. 



I have since tried the method on a number of other clusters 

 ;ind nebulK with uniform success. 



My practice now is to use a very slow plate capable of giving 

 good contrast, and to give it the least possible exposure during 

 contact-printing from the original negative. I use the ordinary 

 Tyro-ammonia developer with half of the normal quantity of 



reproduced copies of M.8 and the Orion nebula, and shall be glad 

 I if you can make use of them. K. D. Naega.mvala. 



' June 8. 



On Mimicry. 

 During the discussion on Mimicry at the last meeting of the 

 Entomological Society, the fact that so many mimetic species are 

 scarce, in comparison with the non-mimetic allies and the models, 

 was brought forward as an argument against the efficiency 

 of mimetic resemblance. Amongst the Indo- Australian Papilios 

 this phenomenon is strikingly illustrated by Papilio alcidinus and 

 catinus, which resemble their models to a surprising degree. As 

 both these rare species deviate widely from their nearest relatives, 

 it occurs to me that the theory of Mimicry, which says that 

 Homoeochromatism and Homoeomorphism between imitating 

 and imitated species are the outcome of selection, can give a 

 satisfactory explanation of the scarcity of individuals of mimetic 

 forms. If we concede for the sake of argument that, for instance, 

 Papilio alcidintis has acquired that wonderful similarity in 

 colour and form to its model, an Uraniid moth, in consequence 

 of a continued selection in the one direction, it is obvious that 



The Great Nebula in Orion. 



Original negative. 



Negative twice re-copied. 



ammonia indicated, and with an excess of bromide. If there is 

 the slightest indication of an image within five minutes, the plate 

 is rejected as too much exposed. The development is continued 

 for about forty-five minutes, with an occasional addition of a 

 little more ammonia ; the plate is, of course, kept rocking all 

 the time in the dark. From the positive thus obtained a second 

 negative is secured, and from it a second positive, and from the 

 last a third negative, which is used for final printing on paper. 

 The method of development is throughout the same as men- 

 tioned above. 



Faint details in outlying portions, for instance, in the photo- 

 graphs of the corona can be very easily brought out in this 

 manner. 



No intensification by mercury or otherwise is employed by 

 me, and I consider it absolutely inadmissible, as it is liable to 

 affect the grains in the neighbourhood of the image, and thus to 

 give a false extension of nebulous matter. The pure process of 

 successive copying and bringing out contrast is perfectly Jegiti- 

 iiiate ; Scheiner and others having shown that the image does not 

 spread thereby. 



I beg to forward for your inspection the original negatives and 



NO. 1442, VOL. 56] 



the result of such a one-sided selection will not only be similarity 

 to the immune model, but also physiological one-sidedness. The 

 more rigorous the selection is, the better will the mimetic species 

 become adapted to its model, and the more will it lose its 

 adaptability to new biological factors. Therefore, when changes 

 in the biological conditions of the area inhabited by the mimetic 

 form take place, such ever-occurring changes as have been 

 described by Wallace in " Natural Selection," the mimetic 

 species, best adapted in one direction, will be at a disadvantage 

 to its relatives which have not been subjected to rigorous, one- 

 sided selection. Consequently, the most striking "mimics," in 

 spite of, or rather in consequence of, the resemblance to immune 

 species, are, in the long run, the less favoured in the struggle 

 for existence, which means that they will become relatively 

 scarce. From this consideration it is apparent to me that the 

 selection of those specimens which are the very fittest in any 

 special direction is in itself a danger to the species, and can lead 

 to destruction. The peculiar bearing which this suggestion has 

 on the theory of Natural Selection, especially on the principle 

 of utility, is evident. 



Zoological Museum, Tring, June 7. Karl Jordan. 



