394 HISTORY OF ASTRONOMY. 



They are found to be singularly free from that, milkiness 

 arising from diffused light and reflected images which 

 invariably exist in the usual construction. The field of 

 view appears strikingly black and impressive. Eamsden's 

 form has been wholly discarded in the positive eye-pieces 

 belonging to the micrometer. These have been made 

 achromatic and orthoscopic. Each is composed of two 

 double cemented achromatics so calculated as to give a 

 perfectly flat field 



In the year 1855, Mr. Spencer constructed an object- 

 glass of nine inches aperture and ten feet focal length, of 

 discs made by Bontemps. From the few trials that have 

 been made with it, its performance is considered excel- 

 lent. Mr. Spencer has also made two object-glasses of 

 5|- inches aperture and seven feet focal length, besides a 

 large number of smaller sizes. In their construction, he 

 has employed discs made by Bontemps, Bader, Guinand, 

 Maes, and Daguet. 



Mr. Spencer has recently received an order for a large 

 heliometer for the Dudley observatory, at Albany, at the 

 contract price of $14,500. The object-glass of this instru- 

 ment is to be of ten inches, clear aperture. 



Mr. Spencer has recently visited the observatories and 

 workshops of England, France, and Germany, preparatory 

 to the opening of a large optical establishment at Albany, 

 in connection with Professor A. K. Eaton and Mr. B. F. 

 Baker. 



