Porz,—4 Methad of measuring Position of Double Stars, 141 
Art. X.—A Description of inexpensive Apparatus for measuring the Angles of 
Position and Distances of Double Stays, and the Method of using it. 
By James H. Porr., 
Plate I. 
(Read before the Otago Institute, 13th August, 1878.) 
UxpousrEDLY anyone who wishes to make observations of double stars 
should provide himself with a first-class telescope equatorially mounted, 
having an aperture of from eight to ten inches; he should place this 
telescope in a commodious and well built observatory and should procure a 
first-class filar micrometer and a galvanic chronograph. He should have 
perfect illuminating apparatus, so that the micrometer wires may appear as 
bright lines on a dark field or as dark lines on a bright field, and he should 
be able at will to employ whatever tint he wishes to give to his field or his 
wires. Besides all this, his telescope should be accurately driven by clock- 
work, so that he may keep a star in one part of the field of view as long as 
he wishes to do so, and may have both hands at perfect liberty to take 
angles of position and to measure the distances between the components of 
double stars. But, unfortunately, this apparatus is extremely expensive. 
Cooke of York will provide every requisite for some £1200; it is not every 
one that can quite see his way to spend such a sum. There are many 
enthusiastie students of astronomy who are anxious to engage in this kind 
of work, but think it quite out of their power to do so on account of these 
same pecuniary difficulties. The following paper attempts to show how 
good work in this department of astronomy may be done at a very trifling 
expense, and to make it evident that the possessor of a good telescope may, 
with a small expenditure of trouble and a still smaller expenditure of money, 
hope to be in a position to take measures of double stars, that will be worth 
preserving in the scientific records of the day. Here I would say, once for 
all, that the methods described in this paper are, many of them, not new. 
Some of them were invented by Sir John Herschell, some by other 
astronomers. For many of the details the writer alone is responsible. For 
working out the mechanical construction, and for many most valuable 
improvements in the water-clock used in the method, the writer has to thank 
Mr. Forsyth, station-master, Caversham. All that the writer claims to 
have done is to have worked out a complete system (the materials for which 
have been derived from various sources), by means of which double star 
observation is placed within the reach of a large class of students of the 
starry heavens, who are debarred from pursuing this fascinating branch of 
astronomy by the great expense involved in procuring the instruments 
ordinarily used in it, 
