342 
ON THE EQUATORIAL TELESCOPE. 
The visibility 
of a star in 
the (lay-time 
is as the mag 
magnifying 
power. 
iiucry. The 
cause? Not 
augmentation 
of sight nor 
darkness of 
ground. 
ments which can be constructed^ and which can be ex- 
pected to be found only in large public observatories, and in 
the private ones of gentlemen of fortune. But even with this 
abatement, the general advantages just now pointed out are 
sufficient to recommend a small equatorial possessed of the 
improvements suggested, to every private academy, and to 
every student and teacher of mathematical and astronomical 
science. 
Query, respecting the visibility of stars in the day-time; 
It is a fact deduced from every observation I have made on 
stars in the day-time, and which, I presume, accords with the 
observations of others, that in proportion as the magnifying 
power of a telescope is increased, it is rendered more easily 
distinguishable, and that without a certain degree of magnify- 
ing power, a star cannot be rendered visible in day-light. 
Query. What is the reason that the magnifying power of 
telescopes produces this effect ? 
In regard to the planets, a probable solution might be given 
from the consideration that the telescope augments the appa- 
rent'size of the object, and presents a larger surface to the 
eye, which is partly the reason why t!:e moon is visible in the 
day-time. But this reasoning will not apply to the fixed stars, 
as no telescope is found to augment their apparent size, or 
make them resemble planetary disks. I am aware, that it may 
be said, that the telescope excludes almost all the light except 
what corAes from the star j and that by increasing the mag- 
nifying power, the ground on which it is seen becomes darker, 
forming a greater contrast to the light of the star. That these 
considerations are not sufficient to account for the effect, I am 
pretty much convinced from the following experiment. I 
have frequently directed a long tube, blackened on the inside, 
with a small aperture at the object end, to a star about sunset, 
when it was easily seen with a very small degree of magnifying 
power, but could never perceive it through the tube, though I 
was 
