3 L 2 Dr. Herschel’s Experiments on the Means 
of an object, and therefore had recourse to the following 
experiments. 
ist Experiment , with the Heads of Pins. 
I selected a set of pins with round heads, and deprived them 
of their polish by tarnishing them in the flame of a candle. 
The diameters of the heads were measured by a microscopic pro- 
jection, with a magnifying power of 80. These measures are 
so exact, that when repeated they will seldom differ more than 
a few ten thousandths parts of an inch from each other. Their 
sizes were as follows: ,1375 ,0863 ,0821 ,0602 ,0425. I 
placed the pins in a regular order upon a small post erected in 
my garden, at 2407,85 inches from the centre of the object 
mirror of my ten-feet reflecting telescope. The focal length 
of the mirror on Arcturus is 1 19,64 inches, but on these objects 
125,9. The distance was measured with deal rods. 
When I looked at these objects in the telescope, I found 
immediately that only the smallest of them, at this distance 
could be of any use ; for with an eye-glass of 4 inches, which 
gives the telescope a magnifying power of no more than 31,5, 
this pin’s head appeared to be a round body, and the view left 
no doubt upon the subject. It subtended an angle of 3", 64 
at the centre of the mirror, and the magnified angle under 
which I saw it was 1' 54 ", 6 . This low power however required 
great attention. 
With a lens 3,3, power 38,15 , 1 saw it instantly round and 
globular. The magnified angle was 2' 18", 9. 
With a magnifying power of 231,8,* I saw it so plainly that 
* The powers have been strictly ascertained as they are at the distance where 
these objects were viewed. 
