1888.] Mr John Aitken on a Monochromatic Bainhow. 137 
with so rich a colour. The rainbow is simply nature’s spectrum 
analysis of the sun’s light, and it showed that on that occasion the 
sun’s light was shorn of all the rays of short wave lengths on its 
passage through the atmosphere, and that only the red rays reached 
the surface of the earth. 
But it will he said, that every object on the surface of the earth 
on that afternoon ought to have appeared red, and nothing hut 
red, if nothing but red rays reached the earth from the sun. ISTow 
this was by no means the case. Everything looked not very 
different from their usual ; they appeared simply tinted with red. 
The reason for this evidently was, that while we received only 
red light direct from the sun, there was a great deal of green, blue, 
and violet light reflected from the sky overhead, and these com- 
bining with the red caused the light to he but little different from 
the usual. The reason why the Ochils glowed with so deep a red 
was owing to their being overhung by a dense curtain of clouds, 
which screened off the light of the sky. Their illumination was 
thus principally that of the direct red light of the sun. 
6. On Neuropteris plicata, Sternb., and Neuropteris 
rectinervis, Kidston. By Robert Kidston, F.E.S.E. 
7. Reflex Spinal Scratching Movements in some Verte- 
brates. By Prof. John Berry Haycraft. 
Many, who have kept dogs, are aware that if the skin covering 
the side of the body be scratched, a dog will move the leg of that 
side as if itself to scratch the part touched. This fact is known to 
the physiologist, and a, so-called, scratching centre, to which the 
sensory impulses are carried, and from which motor impulses to the 
muscles pass, has been shown to exist in the spinal cord. 
I would venture to lay before the Society one or two additional 
facts in this connection which have been observed by me. 
First, as to the sensory area from which this reflex may be 
initiated. The only sensitive part of the skin in most cases is that 
covering the lower ribs in about the middle of their course. If, 
however, the dog be sensitive to irritation, if it has suffered from 
