A NEW DAEK-FIELD MICROMETER FOR DOUBLE- 



STAR MEASUREMENT. 



By a. B. Biggs. 



Eigs. 1, 2, 3, 4. 



I often think it must be very pleasant for the ardent 

 votary of science to liave unlimited means at his command 

 for obtaining such apparatus as he requires in the pursuit of 

 his favourite study ; apparatus elaborately finished, and 

 perfectly adapted for the work for which it is designed. Yet 

 it too often happens that such apparatus becomes a mere toy 

 in the hands of its possessor, he merely contenting himself 

 with its possession, and the enjoyment of its beauties. On 

 the other hand, it remains a fact that some of the grandest 

 achievements of science are due to workers who have had to 

 be content with very simple aad perhaps roughly constructed 

 apparatus, the outcome of their own ingenuity, called forth 

 by the necessities of the case. The writer claims the 

 applicability of the foregoing remarks to his own case only 

 so far as they relate to the necessity of trusting mainly to 

 his own resources in his very limited field of scientific work. 

 The instrument of which the following is a description, has 

 been in this way the outcome of his necessity. Its special 

 function is the measurement of very minute angular distances, 

 such as those of double stars, giving at the same time the 

 angle of position with reference to the meridian. 



A few preliminary remarks on some of the existing forms of 

 Micrometer may help to elucidate the special adaptability 

 of the instrument to be described for the work for which it 

 was designed. The Reticle Micrometer is specially useful for 

 mapping star fields, but a driving clock for the telescope is 

 almost essential. My first Micrometer was of this form, and 

 consisted of a photograph (on thin micro, coverglass) of a 

 scale, ruled on a sheet of glass coated with black paint, and 

 having lines cut through the paint with the point of a pen- 

 knife. The figure was a square subdivided into 400 by 

 parallel lines each way (20 x 20). Each interlinear space 

 was divided by a line running from the centre to the outside 

 of the square each way. The one for use with my highest 

 power is only xVin. square, the spaces between the lines 

 being only 20"oiii- It is, however, quite inadequate for 

 double-star work. 



The Ring Micrometer is adapted for distances occupying a 

 considerable portion of the field, by timing the passages across 

 the ring. But unless the passage describes chords at some 

 distance from the diameter the measures are unreliable. It 

 involves somewhat tedious calculation for differences of 

 declination. 



