HOW TO WOEK WITH THE TELESCOPE. 
471 
In applying these and other tests, some allowance must be 
made for different powers and qualities of eye- sight, as well 
as for the idiosyncrasies of different instruments. Some eyes, 
like some telescopes, possess a remarkably acute defining 
power; others surpass in the power of detecting minute 
points of light, or small differences of shading ; in the appre- 
ciation of colour, again, eyes differ greatly. The observer must 
also be prepared to find his eyesight vary with his health 
or occupations. Lastly, the eyesight will be found to improve 
with practice, a circumstance which must not be forgotten 
when a new telescope is to be tested. 
Sir John Herschel has expressed the opinion, that the 
observation of variable stars offers the most promising field of 
observation to the amateur observer ambitious of effecting 
original discoveries. No part of the heavens can be dili- 
gently observed, even for a few weeks, without detecting 
signs of change. The invention of Dawes's iris- diaphragm 
has greatly facilitated the observation of variable stars. For 
observing the colours, and changes of colours of stars, I think 
a positive eye-piece, having a minute white disc in the focus, 
admitting of illumination through differently coloured glasses, 
carried on a rotating disc, might be employed with advantage. 
For such observations (as, indeed, for all delicate star obser- 
vations) moonless nights must be selected, the opinion ex- 
pressed by Homer of moonlit nights * being quite contrary to 
that held by the telescopist. 
Of nebulae and star- clusters there are many which will 
amply repay close study with telescopes of moderate power; 
though, of course, the observer will not expect to see marked 
traces of configurations with which Lord Fosse's gigantic 
mirror has made us familiar. f Indications are not wanting of 
processes of change in many nebulae. Some appear to be grow- 
ing gradually brighter, others fainter. It is quite within the 
power of the amateur telescopist to detect and estimate such 
variations, to seek signs of changes in position some of the 
nebulae may undergo with respect to neighouring stars, and 
* S’or tv ovpavij} aarpa tyativriv a/JUpi (TtXi'jvrjv 
Waiver apt7rp£7T£Of, &C. 
The lines are beautiful, and have been beautifully rendered by Tennyson 
in a passage which every one has by heart ; but the image is not truthful. 
Pope’s version is gloriously incorrect. 
f It may be necessary to remind some observers, that Sir J. Herschel’s 
pictures of nebulae, having been taken with the front view , must be reversed 
as respects right and left, before they can be compared with other pictures. 
This may be done by holding the picture up to a light and looking at it from 
the back ; or by looking at its image in a mirror. 
