OF ARTS AND SCIENCES. 



387 



by the collodion process, images of the fixed stars to the fifth magni- 

 tude, inclusive, with singular and unexpected precision. 



" The most remarkable instances of their success are the simulta- 

 neous impressions of the group of stars composed of Mizar of the sec- 

 ond magnitude, its companion of the fourth, and Alcor of the fifth 

 magnitude. 



" The following measurements of the angular distance of the com- 

 panion from Mizar were taken from the plates with the aid of the 

 micrometer microscopes of the transit-circle. The distances repre- 

 sent the angles subtended by the images formed at the chemical focus, 

 and measured from the optical centre of the object-glass. 



Plate I. April 27th, 1857 Distance = 14.7 



= 14.7 

 = 14.6 

 = 14.5 

 — 14.5 

 " = 14.1 

 = 14.3 



" In consequence of a difficulty in the way of properly applying the 

 microscope of the transit-circle, without incurring a sensible error of 

 measurement independent of that attributable to the photographic pro- 

 cess, the same plates (with the exception of II.) and six others were 

 subjected to another form of micrometer, hastily arranged for the pur- 

 pose, and less open to similar objection, though still sensibly imperfect. 

 With this the following results have been obtained. 



Ang. of Pos. = 



" = 146° 40' 



" = 148 53 



= 147 55 



Mean = 147 49 



