January io, 1901] 



NATURE 



251 



the water from entering the tube (Fig. i). Five or six strips of 

 tinfoil are now fastened with shellac along the sides of the lamp, 

 leaving a space of from ^ to i mm. between them. The strips 

 should be of about the same width as the clear spaces. They 



*^f^itihs_ 



Fig. 



Fig. 



are to be mounted in two group? tn opposite sides of the lamp, 

 and the rays passing I etween them produce the polar streamers. 

 The proper number, width and distribution of the strips 



Fig. 3, 



necessary to produce the most realistic effect can be easily 

 determined by experiment. A circular disc of metal a trifle 

 larger than the lamp should be fastened to the tip of the lamp 

 with sealing-wax, or any soft, water-resisting cement ; this cuts 



NO. 1628, VOL. 63] 



off the direct light of the lamp and represents the dark disc of 

 the moon. The whole is to be immersed in the tank, with the 

 lamp in a horizontal position and the metal disc close against 

 the front glass plate (Fig. 2). It is a good plan to have a 

 rheostat in circuit with the lamp to regulate the intensity of the 

 illumination. On turning on the current and seating ourselves 

 in front of the tank, we shall see a most beautiful corona, caused 

 I'y the scattering of the light of the lamp by the small particles 

 ol mastic suspended in the water. If we look at it through a 

 Nicol prism we shall find that it is radially polarised, a dark 

 area appearing on each side of the lamp, which turns as we turn 

 the Nicol. The illumination is not uniform around the lamp, 

 owing to unsymmetrical distribution of the candle-power, and 

 this heightens the effect. If the polar streamers are found to be 

 too sharply defined or too wide, the defect can be easily 

 remedied by altering the tinfoil strips. The eclipse is not yet 

 perfect, however, the illumination of the sky background being 

 too white and too brilliant in comparison. By adding a solution 

 of some bluish-green aniline dye (I used malachite-green), the 

 sky can be given its weird colour and the corona brought out 

 much more distinctly. If the proper amount of the dye be 

 added, the sky can be strongly coloured without apparently 

 changing the colour of the corona in the slightest degree, a 

 rather surprising circumstance, since both are produced by the 

 same means. 



We should have now a most beautiful and perfect reproduction 

 of the wonderful atmosphere around the sun, a corona of pure 

 golden white light, with pearly lustre and exquisite texture, the 

 misty streamers stretching out until lost on the bluish-green 

 background of the sky. The rifts or darker areas due to the 

 unequal illumination are present as well as the polar streamers. 

 The effect is heightened if the eyes are partially closed. 



A photograph of one of these artificial eclipses is reproduced 

 in Fig. 3. Much of the fine detail present in the negative is 

 lost in the print, and still more will doubtless go in the process 

 of reproduction. No especial pains were taken to get the polar 

 streamers just right. R. W. Wood. 



University of Wisconsin. 



Sexual Dimorphism. 



I CRAVE your leave to reply to one part of the careful and 

 considerate criticism with which Prof. Meldola has honoured 

 my book on " Sexfial Dimorphism." He declares his inability 

 to see any force in one of the crucial arguments of my theory, 

 namely, that concerning unisexual inheritance, or, in Darwin's 

 phraseology, heredity as limited by sex. He asks. Why should 

 characters mechanically impressed upon the male during a 

 particular period of his life be hereditary, and characters arising 

 i)y spontaneous variation at that period not be hereditary ? But 

 this is not the question we have to consider. The question dis- 

 cussed in my argument is, Why should characters arising by 

 spontaneous variation at any period of life in one sex be inherited 

 by offspring of the same sex, and not by offspring of the opposite 

 sex ? 



Prof. Meldola maintains that on either hypothesis all cha- 

 racters must originate in an individual which must be either 

 male or female, and asks why there should ever be any blend- 

 ing of characters at all. He appears, therefore, to suppose that 

 all spontaneous variations are inherited only by offspring of the 

 same sex as the parents in which they occurred. It is well 

 known that this is not the case. Many instances are on record 

 in which additional fingers have occurred in a man, and have 

 been inherited by both male and female descendants. I have 

 myself much experience in breeding mice, and in them, although 

 the colours of the parents do not blend perfectly or always in 

 the offspring, yet the colour of the male parent is not more often 

 inherited by male progeny than by female. On the other hand, 

 blending often occurs, and if a male pouter pigeon is matched 

 with a female fantail, the male progeny are not perfect pouters, 

 nor the female perfect fantails. 



Darwin has discussed the question in a well-known passage 

 (" Descent of Man," 2nd edit. p. 231) : " If a breeder observed 

 that some of his pigeons varied into pale blue, could he, by long 

 continued selection, make a breed in which the males alone 

 should be of this tint, whilst the females remained unchanged ? 

 I will here only say that this, though perhaps not impossible, 

 would be extremely difficult ; for the natural result of breeding 

 from the pale blue males would be to change the whole stock of 

 both sexes to this tint. If, however, variations of the desired 



