CATALOGUE.' — OPTICAL INSTRUMENTS. 



Qsg 



BiJckmann on tlie colours of sun glasses. 

 Gilb. X. 359. 



Mr. Bockmann recommends, for viewing the sun, an eye- 

 glass composed of four pieces, a light violet, a light green, 

 a dark green, and a dark blue. 



Hornblower on the eye stop of a telescope. 

 Nich. VI. 247. 



On the usual principle. 



Eyepieces for telescopes. Ferguson's lec- 

 tures by Brewster. 

 A cask filled with sand is one of the best supports for a 



telescope. Robison. 



Avslracl of Dr. Herschel's Observation of the Transit of 

 Mercury, with Remarks on the Action of Mirrors. Journ. 

 R.I. II. 64. 



The planet appeared, when viewed with Dr. Herschel's 

 ten feet reflector, and with other telescopes, much darker 

 than any of the solar spots, and perfectly well defined ; no 

 irregularity of form was perceptible at the moment of con- 

 tact ; but the observation appears to have been intermitted 

 at the instant of tie approach of the planet to the sun's 

 limb. Dr. Herscliel could not perceive the slightest degree 

 of ellipticity in the form of the planet's disc. Nothing was 

 gained by employing a higher magnifying power than 130; 

 a circumstance which Dr. Herschel attributes to the effect 

 of the heat of the sun's rays in distorting the figure of the 

 speculums, by paitia' alterations of their temperature; and 

 this supposition he supports by several experiments on the 

 effects of the neighbourhood of heated bodies on their focal 

 lengths. 



Dr. Ilcrschel has also found that, in general, " In order 

 to see well with telescopes, it is required that the tempera- 

 ture of the atmosphere and mirror should be uniform, and' 

 the air fraught with moisture." Thus a frost after a thaw, 

 or a thaw after a frost, will impair the perfection of the 

 focus : a telescope brought out of a warm room into a cold 

 air, or even directed through an aperture of any kind, acts 

 but imperfectly : windy weather is unfavourable to distinct 

 vision, from a mi.\ture of air of difTerent temperatures : an 

 Kiirora- borealis sometimes aiTects the distinctness of the 

 view, as well as the air ascending from the warm roof'Of a 

 liouse : dampness, fogs, and the neighbourhood of moisture, 

 ate very favourable to distinct vision wiih telescopes, ex- 

 cept when a fog is so opaque as totally to intercept it. Dr. 

 Herschel remarks, that some of these obstacles are insu- 

 perable ; but that the effect of heat may sometimes be re- 

 medied by the application of a heated body near the oppo- 

 site surface of the mirror. Y. 



vol.. II. 



In a paper read to the Royal Society, Dec. 1804, Dr. 

 Herschel shows, that the central part of a mirror produces 

 a greater aberration in the image of a fixed star than the 

 whole mirror, and the whole mirror a greater aberration 

 than an annular portion remote from the centre : and that 

 this is true of all good mirrors. 



SeeTheory of Optics, Lenses, Micrometers. 



Micrometers. 

 See Astronomical Instruments. 



Auzout and Hook. Ph. tr. 1665 — 6. I. 123: 

 Townley on Gascoigne's micrometers. Ph. 



tr. 1667. I. 457. 

 Hook on Gascoigne's screw micrometer. Ph. 



tr. 1667. II. .541. 

 Lefevre's micrometer. Mach. A. IT. 103. 

 Kirckius's micrometer. M. Berl. I. 202. 

 Cassini's universal micrometer. A. P. 1724. 



347. 

 Fouchy's micrometer. Mach. A. VI. 45. 

 Smith's optics. 

 Holimann de micrometre. Ph. tr. 1745. 



XLIII. 239. 



Of crape. 



Segner on extending tlie field of a micro- 

 meter. C. Gott. 1751. 1.27. 



Segner on the parallax of a micrometer. 

 C.Gott. 1752. II. 200. 



Short on Servington Savery's micrometer, in 

 1743. Ph. tr. 1753. 165. 



A double object glass. 



Doilond's divided object glass micrometer.. 



Ph. tr. 1753. 178. 1754. 551. 

 Bevis on Gascoigne's micrometer in 1640. 



Ph.tr. 1753. 190. 

 Aopinuson micrometers. A. Berl. 1756. S6j. 

 llaskelync on DoUond's micrometer. Ph. tr. . 



1771. 536. 

 Maskelyne's prismatic micron'.ctcr. Ph. tr. 



1777.799. 



An achromatic prism, sliding along the axis of the tele- 

 scope. Shows the defects of the divided object glass. 



