NATURAL PHILOSOPHY. 171 



glass, flattened and -well-annealed flint glass were employed ; and I 

 believe that, with this modification, the construction of cells would be 

 much simplified, and they might be made at a cost far less than that 

 for which built glass cells can now be obtained. When the angles are 

 formed and the glass joined, the surfaces are ground flat in the usual way. 

 After grinding they are fixed to aflat plate-glass slab with marine glue. 



IMPROVEMENT IN REFLECTING TELESCOPES. 



At a recent meeting of the Royal Astronomical Society, Mr. Grove, 

 F. R. S. made a communication respecting some means he had lately 

 employed with success in improving ordinary refracting telescopes. 

 It is known, that in the object-glasses of these instruments the chro- 

 matic aberration is, as it is termed, over-corrected, while, practically, 

 the spherical aberration is imperfectly or under-corrected. Mr. 

 Grove had tried many expedients to remedy or abate these defects by 

 some simple plan applicable to existing instruments. If the glasses be 

 separated a slight distance, as recommended by Sir J. Herschel, the 

 chromatic aberration is generally increased to such an extent as te 

 greatly deteriorate the performance of the instrument. In some 

 cases where the inner curves of the flint and crown glasses approxi- 

 mate, Mr. Grove had employed with success a highly refracting ce- 

 ment, made of very clear resin and castor-oil, which, acting as a third 

 lens or convex meniscus of a medium dispersing the colored spaces 

 differently from the other two lenses, corrected to a very great de- 

 gree the chromatic without increasing the spherical aberration. 

 This compound forms an excellent tough cement ; it is nearly, and 

 may be absolutely, colorless, and might possibly be used in small 

 telescopes instead of a flint-glass. With telescopes in which the 

 curves would not admit of a cement he had tried lenses of plate- 

 glass, placed at opposite distances between the object and eye- 

 glasses of the telescope ; this plan he conceived was applicable to 

 by far the greater number of common telescopes, and much im- 

 proved their performance. It differs from the plans of Littrow, 

 Rogers, or Barlow, in consisting of a convex lens, and in being ap- 

 plicable, at a very trifling expense, to telescopes constructed in the 

 ordinary way. Mr. Grove had tried various curvatures and dis- 

 tances of this interposed lens, but as his experiments were made 

 for amusement, and with no notion of publication, he had not 

 noted the details. The following was the best result he had ob- 

 tained : In a five foot four inch telescope, having a clear aperture 

 of 3-6 inches, a small plane convex lens of plate-glass, of six feet 

 focus, was placed, at a distance of one foot from the eye-glass, 

 with the plane side towards the eye ; the diameter of this lens 

 need not be more than an inch, and it can generally be at- 

 tached .with ease to the inner extremity of the sliding eye-tube. 

 This had produced so beneficial a result that a mediocre instrument 

 had been changed into a very good one, showing, for instance, the 

 inner ring of Saturn, defining beautifully the division in the main 



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