280 
POPULAE SCIENCE EEVJEW. 
portant discoveries of nebulse and variable stars, by means of 
a telescope only five feet in focal length, mounted on a move- 
able tripod stand. 
Yet, undoubtedly, the feeling experienced by those who look 
through a telescope for the first time is commonly one of 
disappointment. They have been told that such and such 
powers will separate certain double stars, exhibit Sat mads 
rings, Jupiter’s belts, and the continent- outlines on Mars ; 
yet, though perhaps higher powers are applied, the inexpe- 
rienced observer fails to detect these appearances, and can 
hardly believe that they are perfectly distinct to the practised 
eye. And the expectations of the beginner are especially 
liable to disappointment in one particular, which the following 
example will serve to illustrate : — Let us suppose that the ob- 
server’s telescope has a power of 40, and that he proposes to 
apply this power to the examination of Jupiter during the 
last fortnight of July in the current year. He learns from the 
<c Nautical Almanac ” that the diameter of Jupiter will be 
44" -8 ; so that, with a magnifying power of 40, his diameter 
will be 29' 52", or somewhat exceed the minimum diameter of 
the moon. But when the observer comes to apply such a 
power, he will obtain a view — interesting, indeed, and in- 
structive — but very different from what the above calculation 
would lead him to expect. There will be seen a disc, appa- 
rently much smaller than the lunar disc, and not nearly so 
well defined in outline ; in a line with the disc’s centre there 
will be seen three or four minute dots of light, the satellites 
of the planet ; and perhaps the observer will be able to detect 
faint traces of belts across the planet’s disc. Yet neither the 
telescope nor the calculations would be in fault, as the ob- 
server might imagine, and he would find no difficulty in 
proving this ; for during the night of July 26, Jupiter and the 
full moon will be close together, and if they be viewed at the 
same instant, the former through the telescope with a power 
of 40, and the latter with the naked eye, it will be found that 
the centres of the two discs may be made apparently to 
coincide, and that the moon’s disc will not appreciably exceed 
Jupiter’s. Nor should the indistinctness and incompleteness 
of the view be attributed to imperfection of the telescope; 
they are partly due to the nature of the observation and the 
low power employed, and partly to the inexperience of the 
beginner. It is to such a beginner that the following pages 
are specially addressed, with the hope of affording him aid 
and encouragement in the use of the most enchanting of 
scientific instruments, — an instrument that has created for 
astronomers a new sense, so to speak, by which, in the words 
of the ancient poet : 
