1887.] NEW YORK ACADEMY OF SCIENCES. 97 



and one man finished an object-glass whose focal length was 600 

 feet. 



Oassini monnted the objective on the top of a long pole free to 

 move, while the eye-piece was moved along near the ground un- 

 til the object-glass and eye-lens were brought into line with the 

 star to be observed. The tube of the telescope was dispensed 

 with. Hevelius connected his object-glass and eye-piece by a 

 long pole. Newton, in his treatise on Optics, declared that the 

 improvement of the refracting telescope was " desperate," and 

 he turned his attention to reflecting telescopes. But an English 

 optician, named Dolland, about the middle of the 18th century, 

 discovered a remedy. He found that by a combination of lenses 

 of crown and of flint glass he could obtain an almost colorless 

 image at the focus. This, indeed, was a grand victory, and at 

 once enabled the opticians to construct telescopes of less length. 



They could now put more of the magnifying power in the eye- 

 piece, and have a telescope of such a length as could be com- 

 fortably handled. Telescopes, made of such a combination of 

 lenses as we have alluded to, are called " achromatic.^^ As 

 larger and more perfect achromatic telescopes were made, a new 

 source of aberration was discovered, no practical method of cor- 

 recting which is yet known. It arises from the fact that flint 

 glass, as compared with crown, spreads out the blue end of the 

 spectrum more than the red end. The consequence is that two 

 lenses cannot be made so as to entirely get rid of the color. 

 "In a small instrument the defect is hardly noticeable, the only 

 drawback being that a bright star or other object is seen sur- 

 rounded by a blue or violet ring formed by the indigo rays 

 thrown out by the flint glass. If the eye-piece is pushed in so 

 that the star is seen, not as a point, but as a small disc, the 

 centre of this disc will be green or yellow while the border will 

 be reddish purple. But, in the immense refractors of 2 feet 

 aperture and upwards, this 'secondary aberration,^ as it is 

 called, constitutes the most serious optical defect." Some think 

 that no art can cure this defect. Many methods have been 

 tried, all without much practical value. The defect may be 

 lessened in the same way that the astronomers of the 17th cen- 

 tury lessened the effect of "chromatic aberration," — viz., by 



