H. M. Parkhurst on the Photo-mapper. 87 
Art. IV.—The Photo-mapper ; by Henry M. Parxnurst. 
to measure the diameter of a sphere by, removing it to such a 
distance that it should cease to be visiblé and multiplying that 
preciable weight, and multiplying by a constant ; it would be 
analogous to determining the b navies of a star by ascertain- 
ing what proportion of its light is too small to affect the retina. 
e true mode of measurement is by substraction of certain 
known quantities. To illustrate the advantage of this, were 
ing a certain absolute quantity, it would afford usa perfect pho- 
tometer. Or could we construct a glass which should transmit 
polarized light, but which would not transmit common light, 
that would accomplish the same result. But in the mean time 
we may be allowed to designate estimations assisted by mechan- 
ical means as measures of the magnitude of a star. 
' Pp 
arranged that a series of six plates over the object L pom moved 
e bende 
gradually diminished the aperture until the star ceased to 
si 
! a star at a point intermediate between the object-glass. 
Its focus, instead of directly reducing the aperture at the object- 
