Trans. N. Y. Ac. Set. 146 Mar. 20, 



But not only are we able, by the aid of photography, to study the mem- 

 bers of the solar system, but it is possible thus to investigate the rela- 

 tions of the immensely far-off stars, and even to determine their distances. 



In the heavens we discover with a telescope that many stars are 

 changing their positions and distances with respect to their near visible 

 companions ; and investigations have been made proving that some of 

 these revolve about the others as centres, forming systems of their own, 

 like our solar system. 



The measurements required for this work are numerous and delicate, 

 and can be made only on nights v^hen the seeing is Ntry line, by a skill- 

 ful observer. Mr. Rutherford has adopted a method of photo- 

 graphing a cluster of stars, or a double star, in such a way, that the 

 plates can be laid away and measured by a person simply trained in 

 the use of a micrometer. These plates may be taken at large inter- 

 vals. On comparing the measurements, a decision can be reached as to 

 whether any stars have moved with respect to the others. 



Let me describe Mr. Rutherford's method : 



First, a wet plate is exposed for four minutes. This gives stars 

 down to the tenth magnitude. But there may be points on the plate 

 which are not stars ; hence a second impression is taken on the same 

 plate, after it has been slightly moved. All points now doubled aie 

 true stars. Now for measures of arc. After fhis second photograph 

 is taken, the clock is stopped. The now movmg stars down to the 

 fourth magnitude are bright enough to leave a continuous line ; the 

 length of this, in a very accurately known interval, say two minutes, 

 enables the arc to be calculated. Next comes the mapping. The 

 negative is fixed on a horizontal divided circle on glass, illuminated 

 from below. Above it is a system of two rails, along which travels a 

 carrier with two microscopes, magnifying fifty diameters. By the one 

 in the centre, with two cross-wires in the field of view, the photograph is 

 observed. By the other, armed with a wire micrometer, a divided scale, 

 on glass, which is fixed along the rail, is read. Suppose we wish to meas- 

 ure the distance between two stars on the plate. The plate is rotated so 

 that the line which joins them coincides with that which is described 

 by the optical axis of the central microscope (marked by the cross- 

 wires), when the carrier runs along the rails. This microscope is then 

 brought successively over the two stars, and the other microscope over 

 the scale reads the nearest division. Hence then, the fixed scale, and 

 not a micrometer screw, is depended upon for the complete distance. 

 In this way the distance between two stars on the plate can be meas- 

 ured to the Tiffidoth part of an inch. 



By the measurement of such plates, we may hope to add a great 

 deal to our knowledge of the stellar systems. 



