THE MANUFACTURE OF TELESCOPES. 393 



act of injustice not to state their sincere conviction that 

 Spencer's objectives are now the best in the world" 



Mr. Spencer has recently turned his attention to the 

 manufacture of refracting telescopes, and his success in 

 this department promises to be as great as in the manu- 

 facture of microscopes. His principal telescope is one 

 recently completed for Hamilton College, having an 

 aperture of 13^ inches and a focal length of nearly 16 

 feet. The flint and crown discs for this instrument were 

 procured through the agents, Messrs. Cook, Beckel, & Co., 

 New York, from Joseph Bader of Kohlgrub, and have 

 been found to be remarkably exempt from striae. Among 

 the changes that have been introduced in the construction 

 of this instrument, is a method of procuring an absolute 

 optical collimation of the object-glass, combined with the 

 usual means of making this axis coincident with the 

 axis of the tube. The method used to produce this re- 

 sult enables the observer instantly to detect the existence 

 of any error in respect to collimation ; and a further ad- 

 vantage of the construction is that the object-glass may 

 be made to take any angle of position to the declination 

 axis, or to a line joining the components of a system of 

 double stars. The focal length of the object-glass is 

 unusually short for its aperture ; and to increase its mag- 

 nifying power, an equivalent of twice its focal length is 

 obtained by the introduction of a negative achromatic 

 near the eye-piece. The higher of the six negative eye- 

 pieces that belong to this instrument are solid, and of a 

 construction radically different from any heretofore used. 



