to the determination of Astronomical data. 27 
proportionate increase in the time of exposure ; 
work of measuring the photographic images and in reducing 
the measurements, an i, i ivi ! 
labor performed, I take the liberty of stating my own estimate 
of the two methods of observing double stars, viz: the photo- 
graphic method and that of direct measurement with a filar 
micrometer or heliometer. In the case of double and triple 
stars I have no doubt that the better method is that of direct 
measurement. The labor of setting the circles and finding the 
star is common to both methods, but during the time required 
graphic images a practised observer would make and reduc 
series of direct measurements. It is thus possible by the direct 
method more easily to repeat the observation under varie 
conditions of atmosphere, observer and instrument, and in this 
way to render the final result less liable to systematic errors. 
Tt is true that according to Prof. Bond’s calculations, the photo- 
eraphic method is decidedly the more accurate, but some ex- 
os istrustful of inferences drawn 
to avoid systematic error, or to give it as much as possible the 
nature of those irregular errors which in the long run tend to 
eliminate themselves. Finally, it may be stated as a general 
