NEW OBSERVATIONS OF THE PLANET MFncURY. 4J^5 
Those magnifying from 135 to 170 diameters turned out the bo;it on (lie 2M!ich ; on 
the 6-inch, from 75 to 135. With 300 on the 21-inch, if the air \v;is g()<"l, the iiiajk 
ino;s still showed fairly \vell. 
In place of low powers, diaplirngming the emcrgoui prncil — a device of Mr, 
Douglass — sharpens the detail. l^otli processes liavc infon^^ling j)hvsiulogical 
beariniirs. 
O 
Lastly comes persistence. No air is contimiously good. It lins good momenta 
and bad. The good must be patiently waited for and seized. ^Vlla( ap]ili<« to a 
single sight applies a forimi to acrpiisition of any full knowledge of the plaix't; nnd a 
little persistent practice will bring a great surprise, greater than tlie surprise uff^^fMug 
it all, — the surprise that it was not all seen before. 
5. Method of Measurement. — The mode of taking the measures also deserve- 
a word. My measures were made with a filar micrometer, the thread IjcJiig phiccd, 
not on, but near, the edges of the disk, without and within respectively. This metljod 
has great advantages over any attempt to place the thread on the limb; in f .* t, lo 
ace the thread on the limb accurately is impossible, first, because of tiic oxc( ,--ive 
irradiation produced by so doing, and secondly, because the eye is iniablc to hcc v-'-ll 
both thread and limb at the same time. 
For accuracy the filar micrometer is to be preferred to the doulde iiua^ mi- 
crometer. There is prevalent a curious mistake about the latter, to the ciTcct ihnt 
it eliminates irradiation. That this is not possible is en-ily shown by considering 
that contrast alone causes irradiation, and that the contrast in the ca.se is always 
between the disk and the sky. The error seems to have arisen from supposing tin' 
radiation to be due to contrast bet 
y the c} 
of course is not the case. The only reduction of irradiation in the d 
micrometer comes from the somewhat fainter images it gives, an effect 
produced with the filar micrometer by slightly reducing the li 
glass. Thus the double imacre micrometer possesses on this 
ca>i]v 
posted 
he filar micrometer. On the other hand, it possesses very distmcL 
for the images formed by the double eyepiece are subject to greater 
d fiare, — a distortion which renders the 
able 
6. Two 
The observations themselves consisted of 
classes : (1) drawings of the planet's surface features ; (2) micrometric mensure^ of 
the planet's diameters and discussion of the same. 
7. Drawings. - We will take up the drawings first. 31 1 .1 . .■uvin,^'. and 1 2 ..ketcl, 
were made of the planet's di,sk. Of these, 104 drawings and .-■, skHches were n,a, 
.- V 
