582 iJiBLioGRAPnY OF iierschel's writings. 



Herschel, W.: Syxopsis of the WraxiNGS or— Continued. 



A. D. Tol. P. 



action of Mirrors. By William Herschel, LL. D., F. R. S. Read 



February 10, 1803. . 

 1803 93 214 The observatious were made (1802, Nov. 9) with a glass mirror of 7 



feet focus and 6.3 inches aperture under favorable conditions, and 



215 also with a 10-feet reflector. Eye-pieces made of dark-green glass 

 and others smoked on the side next the eye were used. 



216 No ring about the planet ; no distortion of the limbs of the sun or 



Mercury at egress. 



217 Mercury was perfectly spherical in figure. 



217 Observations and exj,eriments relating to the causes ivhich often affect mir 

 rors, so as to prevent their shotving objects distinctly, 



217 The experience of many years will enable me to assign the principa 



causes of disappointments in the use of mirrors. 



218 The following observations have all been made with specula of un- 



doubted goodness, so that every cause which impeded their jiroper 

 action was extrinsic. 



218 Moisture in the air. Damp air is no enemy to vision. 



219 Fogs. 



220 Frost. 



221 Hoar-Frost; Dry- Air. 



222 Northern-Lights; Windy-Weather; Fine in Appearance. 



223 Over a Building; The Telescope lately brought out. 



224 Confined Place; Haziness and Clouds. 



225 Focal-length. 



226 In order to see Avell with telescopes it is required that the tempera- 



ture of the atmosphere and mirror should be uniform and the air 

 fraught with moisture. 

 AU the preceding observations are accounted for by this principle. 

 228 Experiments [on the effects of change of focal length of a speculum. ] 

 Heat applied back and front of a glass speculum lengthened the 

 focus. 



230 With a metal mirror the focus became first shorter and then length- 



ened. 



231 In observing the sun similar effects may arise. 



232 Perhaps these might be counteracted by an application of heat to 



the back of the mirror or by an interception of it in front. 



1803 93 339 Account of the changes that have happened during the last Twenty-Five 

 Years in the Belative Situation of Double-Stars ; tvith an investigation of 

 the Cause to which they are owing. By William Herschel, LL.D., 

 F. R. S. Reai June 9, 1803. 



339 In Phil. Trans., 1^502, p. 477, I have defined insulated stars. The dis- 



covery of [Ceres and Pallas'] has enlarged our knowledge of the 

 system of insulated stars. 



340 I have aheady shown that two stars may revolve about their common 



centre of gravity; and that it is probable there would be many 

 such binary systems among all the stars of the heavens. But 

 neither of these reasons is a proof of the actual existence of such 

 systems. I Avill here give an account of observations which will go 

 to prove that many of them are not merely double in appearance, 

 but are real binary combinations, intimately held together by the 

 bond of mutual attraction. 



341 In Plate VII, Fig. 1, call the place of the sun, O, of the larger star 



of a double star, a, of the smaller, x- 



