November 17, 192 1] 



NATURE 



369 



case the variation or mutation is produced without any 

 change of environment, and often persists under a 

 great range of difference in environment. We call 

 such a change due to difference in germinal factors a 

 mutation or congenital variation, a change due to 

 environment or modification an acquired character. 

 Therefore, to assert, as Prof. Goodrich and Sir Arch- 

 dall Reid do, that all characters are acquired during 

 the lifetime of the individual is a misuse of terms, 

 and tends to confuse and obscure a perfectly clear dis- 

 tinction. Characters due to genetic factors are 

 developed, not acquired ; characters due to environ- 

 mental change without change of genetic factors are 

 acquired. 



Prof. Goodrich states that in an environment which, 

 on the whole, alters but little, evolution progresses by 

 the cumulation, along diverging lines of adaptation, 

 of new characters due to mutation. I regret to have 

 to disagree fundamentally with a zoologist for whose 

 work in his own special field I have so great a respect, 

 but my own studies have led me to the conclusion that 

 this statement is not in accordance with our present 

 knowledge. What we know of mutation and adapta- 

 tion seems to me to indicate very strcmgly that they 

 have nothing whatever to, do with each other. 



J. T. Cunningham. 



University of London Club, 21 Gower Street, 

 November 2. 



Reflection " Halo " of (Semi-) Cylindrical Surfaces. 



I RECENTLY noticed on the wall of a staircase in 

 this college a clear ring of light, produced by a low 

 sun shining through an open window. The ring was 

 free from colour, circular so far as could be judged 

 by eye, and complete except towards the bottom, 

 where it was cut across by the shadow of a polished 

 wooden handrail. A little investigation showed that 

 the appearance was due to the reflection of sunlight 

 falling on this rail, the upper surface of which is 

 more or less cylindrical. Sighting along the rail 

 showed that the axis of this cylinder passed approxi- 

 mately through the centre of the ring. The pheno- 



FlG. I. — AP = incident ray ; PB = refle:ted rav. Broken lines are projections 

 on XZ plane of these rays; broken ring shows form of "halo" on a 

 screen parallel to the VZ p ane. 



menon must have been observed often enough, but 

 I have not seen it described. It is readily treated 

 by simple geometrical methods as follows :— 



Let the reflecting segment of cylinder considered have 

 its centre at the origin, and let its radius be r and its 

 axis that of X (Fig. iV Let an incident ray AP parallel 

 to the XY plane fall, from the -X direction, on the 

 point P(o,y„,2j of the cylinder ; the equations of the 

 ray may then be written y+wx=,yo, i; = *:o- The normal 

 NO. 2716, VOL. 108] 



at P is z^y=y^z, and the perpendicular plane through 

 the origin is y„y+2,z=o. The incident and reflected 

 rays must cut this plane in two points, the join of 

 which is bisected by the origin. The incident ray 

 meets it at 



(yo+^y»», -v/y.-^-o) 



so the point with equal and opposite co-ordinates must 

 lie on the reflected ray ; so does Y{p,y^,z^. Hence 

 the reflected ray is given by 



From this it follows at once that (v — yo)^+(s — 2^)* = 

 m-x^. Thus, considering all values of y^ and z^, we 

 see that so long as y„ and z^ axe small compared 

 with X, y, z, the reflected light is practically a right 

 cone, with its axig along that of the c\iinder. The 

 distribution of brightness and other points can easily 

 be worked out, but would make this note unduly 

 long. 



The phenomenon is shown by any reflecting cylin- 

 der ; a nickelled shaving-soap tin answers admirably. 

 The radius of the ring thrown on a screen normal 

 to the axis is readily shown to be practically inde- 

 pendent of the diameter of the cylinder, but propor- 

 tional to the tangent of the angle between the incident 

 rays and the axis, as the final equation shows. The 

 ring stops at z=±r on the — Y axis — i.e. appears to 

 be cut across by the shadow of the cylinder. In- 

 crease of length of the cylinder, of course, results in 

 a widening of the ring. J. H. Shaxbv. 



University College of South Wales and 

 Monmouthshire, Cathays Park, Cardiff, 

 October 31. 



Microscope Illumination and Fatigue. 



A VERY large part of the fatigue produced by work- 

 ing at the microscope for long hours is due to the 

 use of incorrectly adjusted illumination, whether too 

 bright or too weak. In routine work of a critical 

 nature, where daylight is out of the question fcM- 

 several reasons and an artificial light source must be 

 used, the light may be regulated so as to be satis- 

 factory for the combination of eyepiece and objective 

 most frequently used, but any change of either 

 materially alters the brilliancy of the field. Any 

 alteration of the substage to correct for this will upset 

 the critical adjustment of the optical system ; the 

 changing of light-filters is a rough-and-ready solution, 

 but fine differences cannot easily be made without 

 varying the quality of the light, and, in most cases, 

 require the removal of the eye from the microscope 

 to carry out. 



When working with a 30-c.p. " Pointolite " lamp 

 (which provides the ideal homogeneous source of 

 light, and with a suitable condenser and mono- 

 chromatic filter gives more than enough light for 

 anv combination), it was found that the excess of 

 light was ver\' tiring to the eyes, and the manipula- 

 tion of extra filters for each change of eyepiece or 

 objective involved a great loss of time. It seemed 

 possible that the pyrometric lamp advertised by the 

 same makers might produce better results, as the 

 candle-oower can be varied between ver>- wide limits ; 

 but before trying it experiments were made with the 

 30-c.p. lamp. 



It was found that by inserting extra resistance into 

 the arc circuit (which in this case was some 350 ohms) 

 the light may be varied from full to a dull red glow, 



