320 Sir William Herschel’s observations and experiments 
experiment upon many different large and small stars, they 
all concur to show that 0.21 does not sensibly stop any light; 
but that less does certainly render the object rather less 
luminous ; so that the opening may be put at two-tenths of 
an inch in my eye. 
VIII. Application of the extent of natural and telescopic vision to 
the probable arrangement of the celestial bodies in space. 
When the extent of natural and telescopic vision is to be 
applied to investigate the distance of celestial objects, the 
result can only have a high degree of probability ; for it will 
then be necessary to admit a certain physical generic size 
and brightness of the stars. But when two hypotheses are 
proposed to explain a certain phenomenon, that which will 
most naturally account for it ought to be preferred as being 
the most probable. Now as the different magnitudes of the 
stars may be ascribed to a physical difference in their size and 
lustre, and may also be owing to the greater distance of the 
fainter ones, we cannot think it probable that all those of the 
5th, 6th and 7th magnitude, should be gradually of a smaller 
physical construction than those of the 1st, 2d, and 3d; but 
shall, on the contrary, be fairly justified in concluding that, in 
conformity with all the phenomena of vision, the greater 
faintness of those stars is owing to their greater distance from 
us. The average size and brightness of several stars of the 
first magnitude being also taken as a standard, in the manner 
that has been shown, the conclusion drawn from different 
series of equalisations will support one another; so that we 
shall be able to say a distant celestial object is so far from us, 
