1887.] NEW YORK ACADEilT OF SCIENCES. 103 



however, do not support this view. No European optician will 

 claim to do better work than the American firm of Alvan Clark 

 & Sons in producing uniformly good object-glasses, and this firm 

 always does the work by hand, moving the glass over the polisher 

 and not the polisher over the glass." 



Little imperfections are sure to exist after the first polishing. 

 It is in the nice correction of these that the great skill of the 

 optician is shown and much time is consumed. 



The American firm of Alvan Clark & Sons enjoys the rej)uta- 

 tion of being the best opticians in the world for polishing large 

 lenses. 



When the Russian government decided to construct a telescope 

 that would surpass in size the great Washington telescope of 26 

 inches diameter of object-glass, Otto Struve, the director of the 

 Imperial Ob&ervatory, was commissioned to make an examina- 

 tion of the optical workshops of the world to discover where the 

 best object-glass makers could be found. After he had made a 

 thorough examination in Europe and in this country, he gave 

 the contract to the firm of Alvan Clark & Sons. This great 

 object-glass 30 inches in diameter is now completed. When the 

 Russian glass was contracted for, the trustees of the Lick Ob- 

 servatory in California ordered an object-glass of 36 inches aper- 

 ture — so that we still have in the United States tlie largest 

 refracting telescope in the world. 



I have dwelt upon the object-glass of a great refracting tele- 

 scope because of its vital importance. We will have some expla 

 nations to make in regard to mountings, etc., when at the close 

 of the lecture we throw on the screen several pictures to illus- 

 trate our subject. 



As we have previously stated, Isaac Newton, in the latter part 

 of the 17th century, believed that there was no remedy for the 

 defects in the refracting telescopes as then made, and he turned 

 his attention to reflecting telescopes. Now it is well known that 

 when parallel rays of light fall on a concave mirror they will all 

 be reflected back to a focus, there forming an image of the object 

 from which the rays emanate. The form of this concave mirror 

 must be such that a section of it cut by a plane parallel to the 

 length of the telescope will be a parabola. The image formed 



