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 dimensions. 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. 



