■ friK. . 



1 



for the Advaiicemen t of Science. 4 1 7 



direct image F to the side of the lube into such a position that it could 

 be received directly in the eye-piece. But it was found that correct 

 images, could not be produced in this oblique position by speculums of 

 the ordinary form, — and as yet a means o[ grinding them to a form that 

 would give correct oblique images had not been successfully devised. 



He had received numerous and some most ingenious suggestions for 

 placing the observer in such a position as to view the image F directly 

 . at the middle of the tube, . One difficulty to be got rid of was, that the 

 head and person of the observer would itself abstract much of the 

 light that should be permitted to proceed to the speculum to be there 

 reflected to form the image. Another was, that the temperature of 

 the body of the observer, so much higher as it was than the surround- 

 ing air, tended to produce ascending currents, which both produced a 

 wavering motion and an incorrectness of the image quite fatal to the 

 accurate performance of the instrument. These could, perhaps, be 

 guarded against, — and he had received many most curious and some 

 very ingenious plans for disposing even of the entire body of the ob- 

 server by lying along in a tube properly and well-ventilated with means 

 for conveying away the breath and heated air out of the telescope tube 

 without interfering with the air in the tube itself. But one element in 

 the problem seemed most strangely io have been overlooked by all, 

 and yet he was bold to say it was the most important of any ; he al- 

 luded to the diffraction caused by the head of the observer, or by the 

 box or case in which it was proposed to encase it or himself. The 

 effect of this diffraction upon the performance of the instrument would 

 become more injurious also the larger the profile of the object which 

 stopped off the light. Under all these difficulties, he had come to the 

 conclusion that all attempts at viewing the direct image must be aban- 

 doned. He then turned his attention to prismatic reflexion, in which 

 comparatively much less light was lost. But he found it impossible to 

 obtain large and sufficiently well-formed prisms to insure the best ac- 

 tion. But having obtained a very fine prism of a small size by a dis- 

 tinguished optician, he determined to try it. This required the prism 

 to be placed much nearer to the apex of the cone of light reflected 

 from the great speculum (F), and then the eye-piece required to be of 

 such a form as greatly to diminish the field of view, a matter of the 

 niost serious import in the researches of the nebulse in which he was 

 engaged. This difficulty, however, he had surmounted by first using 

 the larger reflector (B) to examine an extensive field of view, though 

 less perfectly, — and then, by a simple contrivance, turn it aside out of 

 the cone of light, and more minutely examine the limited field of view 

 afforded by the prismatic reflexion. Yet after all, he found this most 

 injurious to the work of making accurate drawings of the appearances. 

 But another still more serious difficulty is fatal to this mode of re- 

 flecting,— namely, that although guided by the research of M. Jamin, 

 and using the formula afforded by Cauchy, and every other formula he 

 could conceive or arrive at the knowledge of, both as to polarized light 

 and aberration, he had altogether failed in making this eye-piece achro- 

 »«atic ; and if any mathematician of the Section could supply hm with 

 the means of securing this result, he would feel himself under great 

 obligations to him, and would cheerfully furnish him with the precise 



