110 Mr. L. Wright on Mica Films and 
paper-cutter flat down to it. In this simpler way I have split 
many ^-wave sheets, measuring, say, 13 inches by 5 or 7 inches 
wide, and plenty of smaller sheets, which by reflection only 
show nine or ten Talbot lines throughout the whole spectrum. 
Coloured films could probably only be obtained by Mr. 
Madan's method. Even the smooth paper-cutter will cause 
signs of bruising or scratching; but this is of no conse- 
quence, as every mark totally disappears svhen cemented with 
balsam. At the same time I should much like to know how 
the trade splitters manage to get their sheets apart without 
this. The best mica I have found was Indian. Mica must 
sometimes occur perfectly colourless, as Mr. Fox gave me 
a small sample ; but where more can be got no one seems 
able to say. Such mica would be a great boon in mica- 
selenite or crossed-mica combinations ; but what the trade 
call " white " mica is simply ordinary mica which, owing to 
countless air-bubbles, shows pearly iridescence. When the 
original slab is split into sheets the mica " axis " can be most 
carefully ascertained and marked on one of these, and then 
transferred without further trouble to all the others. 
Since my paper was published I have devised designs 
showing, even more beautifully than the " optical chromo- 
trope," the rotational colours of circularly-polarized films. 
In their startling kaleidoscopic changes on the screen they 
almost baffle description, and would certainly puzzle any one 
not thoroughly acquainted with the subject. To such it will 
be sufficient to say that they consist practically of two or 
more superposed circularly-polarized designs, planned to go 
thus together, in appropriate positions. As design, polarizer, 
or analyzer is rotated, the variety of effect is almost endless, 
and beyond any thing I have seen yet in spectacular de- 
monstration. 
One word further as to prisms. In the note to his able 
paper, Mr. Glazebrook apparently refers to a few remarks I 
made at the Meeting of the Physical Society where it was 
read. In those remarks I referred expressly to the paper by 
Hartnack and Prazmowski, of which a translation had been 
sent me by the Rev. P. R. Sleeman ; and I am glad to 
see that this fine paper has at last been published in English 
in the current number of the Journal of the Royal Micro- 
scopical Society. It seems strange that Hartnack should have 
chosen the plane of section, and not direction of the ray, at 
right angles to the optic axis, as the best theoretic position, 
since, without Mr. Glazebrook's elegant demonstration, it 
appears at once that the latter position must give even the 
greatest difference in refractive indices — the main object Hart- 
nack had in view. Some years ago I asked Messrs. Darker 
