of Edinburgh, Session 1870 - 71 . 
463 
evening parties, to which men of science and amateurs were in- 
vited, and it was the custom to exhibit scientific novelties, and to 
make them the subject of conversation. 
On the evening in question I brought as my contribution to the 
meeting some very thin films of glass (such as are shown in glass- 
houses to visitors by a workman, who blows a portion of melted 
glass into a large balloon of extreme tenuity, and afterwards 
crushes the glass to shivers). Such a film of glass I brought to 
Dr Wollaston and his friends, and after showing that in the well- 
lighted apartment it displayed a uniform appearance without any 
markings, I removed it into another room, in which I had prepared 
a spirit lamp, the wick of which had been impregnated with com- 
mon salt. When viewed by this light, the film of glass appeared 
covered with broad nearly parallel bands, which were almost black, 
and might be rudely compared to the skin of a zebra. Similar 
bands, but much fainter, were seen by transmitted light. All pre- 
sent agreed that this curious phenomenon could only be due to the 
extreme homogeneity of the light of the lamp with the salted wick, 
which much exceeded any previous estimate of it. It did not 
occur to any one that evening to procure a lens and a plate of 
glass, in order to try the effect of the light on Newton’s rings. 
But such an experiment tried soon afterwards revealed an astonish- 
ing augmentation of the number of rings visible. I followed up 
this observation by publishing a paper in 1826 (Brewster’s Journal, 
vol. v. p. 77), in which I determined, among other things, the fol- 
lowing facts, namely, that all the salts of soda gave the yellow line 
D, which I therefore affirmed to be characteristic of sodium. That 
the salts of potash give a violet light, together with a single red 
ray situated almost at the end of the spectrum, and with no other 
light near it. [Subsequently Brewster made careful observations 
upon this ray, and found it to be coincident with A in the solar 
spectrum, a remark wdiich recent researches with more powerful 
instruments have shown to be not entirely exact. Brewster did 
one great service in pointing out the fact that in inquiries like this 
an achromatic telescope is not necessary.] 
The following is a quotation from this paper (vol. v. p. 77): — 
“ The flame of nitre contains a red ray of remarkable nature. This 
red ray possesses a definite refrangibility, and appears to be cha- 
