AMERICAN INSTITUTE. 415 



tlieir course more than others ; when the image of an object, seen through 

 it was indistinct, compressed and encircled by a colored ring. This was 

 for years the chief and most formidable obstacle presented to the manu- 

 facturers of telescopes, who brought to their aid mathematicians, artists 

 and chemists ; all of whom for a long period of ti iie, believed and reported 

 that achromatism was utterly impossible, and that light could not be de- 

 flected, without being decomposed. Even the great men were led to this 

 conclusion, by imperfect and badly conducted experiments. Subsequent 

 discoveries have proved that the rays of light may be curved without being 

 separated. But notwithstanding the progress that has been made in the 

 theory of light and colors, the subject of achromatism is still one of the 

 most embarrassing in regard to theory and practice. Plato considered it 

 to be an usurpation of the rights of God, to attempt the investigation of 

 this mystery of nature. At length Sir Isaac Newton brought his great 

 mind to bear upon the subject, and demonstrated, by many decisive experi- 

 ments, that color depends not on any modification of light acquired by 

 reflection or refraction, but is inherent in the light itself; the solar rays 

 being composed of rays of all the colors contained in the spectrum, which 

 are difi'erently afi'ected while passing through refracting medea. The merit 

 of the discovery of achromatic compensation is tiow supposed, and gener- 

 ally conceded to belong to .John Dolland, who reasoning from the construc- 

 tion of the eye, which is a perfect achromatic instrument, and skillful ex- 

 periment tend to the practical improvement, and the subsequent discoveries 

 of Fraunhofer have opened up a new view of the composition of the spec- 

 trum. 



The vision of telescopes must always be more or less impaired by natural 

 occurrences that are forever happening. Every living animal moving on 

 the earth causes vibrations, some of which were so fine as to be utterly un- 

 known before high magnifying powers incontestibly proved their existence. 

 Early on any Sunday morning, you may place your ear close to the ground 

 at Fourth street, corner of the Sixth avenue, and hear the first car leave its 

 depot at Forty-first, which if there were no other occurrences of the same 

 character, happening at the same time, would efi'ect the high powers of a 

 telescope at Fourth street. Thunder, percussions, wind, roaring of the 

 ocean, are always occurring, so that the surface of the earth is never free 

 from vibrations, sufiicient to disturb the passage of light through telescopes, 

 and cause the heavenly bodies to twinkle into large diameters, and this will 

 limit high powers. 



A water telescope has been invented arranged for two eyes, with which 

 instrument objects have been distinctly seen on the bottom of the ocean 

 whe e the water was seventy-two feet deep. 



The reason why we rarel}^ see the bottom of a clear lake, or sea, where 

 the depth is within the power of vision, is not that the light is reflected 

 from objects at the bottom so feeble as to be imperceptible to us from their 

 passage through the dense medium of water, but from the irregular refrac- 



