re 
HIS EXPERIMENTS. 283 
- The author relates first, that in 1774, he endeavoured 
to ascertain experimentally, with the naked eye and at 
the distance of distinct vision, what angle a circle must 
subtend to be distinguished by its form from a square of 
similar dinrensions. The angle was never smaller than 
2/17”; therefore at its maximum it was about one four- 
teenth of the angle subtended by the diameter of the 
moon. 
Herschel did not say, either of what nature the circles 
and squares of paper were that he used, nor on what 
background they were projected. It is a lacuna to be 
regretted, for in those phenomena the intensity of light 
must be an important feature. However it may have 
been, the scrupulous observer not daring to extend to 
telescopic vision what he had discovered relative to vision 
with the naked eye, he undertook to do away with all 
doubt, by direct observations. 
On examining some pins’ heads placed at a distance in 
the open air, with a three-foot telescope, Herschel could 
easily discern that those bodies were round, when the 
subtended angles became, after their enlargement, 2’ 19”. 
This is almost exactly the result obtained with the naked 
eye. 
When the globules were darker ; when, instead of pins’ 
heads, small globules of sealing-wax were. used, their 
spherical form did not begin to be distinctly visible till 
the moment when the subtended magnified angles, that 
is, the moment when the natural angle multiplied by the 
magnifying power, amounted to five minutes. 
In a subsequent series of experiments, some globules 
of silver placed very far from the observer, allowed their 
globular form to be perceived, even when the magnified 
angle remained below two minutes. 
