AT + Scientific Proceedings, Royal Dublin Society. 
and index the angle could be measured to tenths of a degree. 
When two bright objects were viewed through this instrument 
two images of each were formed by the double-image prism, either 
of which by turning the Nicol could be made as faint as was 
desirable. Whatever their relative light be, the faint image of 
the brightest could thus always be reduced to equality with the 
bright image of the faint object. The relative brightness is then 
deduced from the angle through which the Nicol is turned. This 
form of photometer may be used without a telescope for the com- 
parison of bright stars near each other, but the loss of light is large. 
Another one was constructed in which the Nicol and prism were 
both placed in front of the eye-lens, the Nicol being next the eye. 
A much simpler arrangement consisted of two concentric tubes, 
one carrying a graduated circle, the other two indices; in the 
first of these tubes a double-image prism was inserted, the other 
which was held next the eye carried a Nicol. This photometer 
was used without a telescope to compare the relative brightness 
of Saturn and Mars, and Jupiter and Venus. It was later on 
attached to the eye-end of a telescope, and a Rochon prism in- 
serted instead of the double-image prism of spar. The prism 
being of quartz, the separation of the images amounted to less 
than 1°, so that the emergent pencils overlapped each other by 
nearly three quarters of the diameters of each. The apparatus 
had moreover the great advantage that the images were precisely 
alike, and nearly achromatic. A combined spectroscope and pho- 
tometer was also constructed for comparing the colours of the 
components of double stars, by measuring the relative light of 
different portions of their spectra. 
These photometers could only be used for comparing objects 
very near together, such as double stars or satellites. For some- 
what greater intervals two achromatic prisms of small angle were 
placed in front of a telescope, covering the central part of the 
object glass. Two images of any object would thus be formed, 
separated by an interval dependent on the angle of the prisms, 
and on their relative positions. 
All the photometers described are open to the objection 
that the loss of light is very great, from 60 to 80 per cent. 
This was especially felt during the observations of the satellites 
of Mars, and led to the invention of another class of photometers, 
