804 PROFESSOR PIAZZI SMYTH ON COLOUR. 
of the little spectroscope, so as to bring one colour after another of its miniature 
spectrum of the illuminating lamp-flame, upon the said reflecting point. 
In an instant the success of the proceeding was found practically perfect. 
For at any point of the spectrum of observation in the eye-piece, the observer, 
everything else being correct, could either make or unmake a fearful amount of 
parallax of the illuminated point accordingly as he illuminated it with a different, 
or the same, region of the illuminating spectrum. Either day-light, or lamp- 
light, and that last no more than a little flame of a sponge-lamp, burning only 
an ounce of rock oil through a whole day, will quite suffice for the illumination’s 
origin.* While the number of spectral tints procurable thus for the point, one’ 
after the other, and by the easy plan of delicate turns to a fine screw,—proved 
almost a fascinating employment ;—especially when the result completely meets 
the scientific object proposed, viz., the abolition of parallax for an illuminated 
reference-line in any part of a telescopic spectrum under examination. 
Or, if there is one feature about the arrangement not yet fully perfect, it is 
merely that green light is not reflected very decidedly from steel. But that 
imperfection, Mr HiuceEr is at the present moment rectifying by making another 
pointer of the almost equally hard, far more anti-oxidisable, and much whiter 
metal, Iridium.t And so the trouble of three years has at last been successfully 
closed, by practically acknowledging,—that however a pigment, or a stained 
glass, may approach by eye-matching to a spectral tint, its physical action may 
be totally different, if it has any different prismatic refrangibility to the part 
of the spectrum being observed: and it may have something in that physical way 
very different indeed. 
Paw! 
Eprnpureu, Feb. 21, 1879. 
* In my own spectroscope, this little flame being 12 inches from the illuminating arrangement 
has its image re-formed there by a common magnifying glass of 3 inches solar focus, placed half-way 
between. 
+ As this metal, or rather its alloy with Platinum, as Iridium-platinum, promises to fulfil a most 
important part in forming the “ finder” and “ reference-spectrum’s” small mirror in front of a spectro- 
scope’s slit, as well as to serve for ruling diffraction “ gratings” upon,—the following scrap of recent 
practical information about it, from Mr Hilger in his workshop, may perhaps usefully occupy this 
last corner of the page :— 
“ Tridium-platinum,” he writes in March 1879, “is a very fine metal, but rather very dear. You 
may easily calculate what a small mirror of one or more inches would cost, as the ounce of it is charged 
thirty-five shillings!! The specific gravity is about the same as platina. A piece I bought the other 
day, $ inch thick by 1°5 inch square, rather full in size, came to £13. I almost fainted at such an 
awful price: still it was ordered, and I had to pay for it. So far as polish is concerned, it does get as 
white and bright as a silver mirror, and never tarnishes in the least. I have already polished several 
small mirrors of iridium-platinum, and it answers perfectly.” 

