HISTORY OF PHYSICAL ASTRONOMY.

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solar light, and to increase the quantity gradually, until the visual organbecomes so habituated to the effulgence of the light as to he capable ofviewing with impunity the whole of the solar disk. Galileo was equallyignorant of any method of observing the sun except that which consistedin taking advantage of his horizontal position when rising or setting. Ina postscript added to his second letter to Welser, he alludes to an immensespot which appeared on the centre of the sun’s disk, and which he, aswell as many others to whom he shewed it, was enabled to perceive withthe naked eye at sunset, on the 19th, 20th, and 21st of August, 1612*.Schemer appears to have been the first observer who reduced intopractice the method suggested by Appian. In a dissertation on the solarspots, which he published in 1612, he remarks that the sun may beviewed at any altitude by placing before the telescope a piece of glass, ofa green or azure colour. He adds that this is the method practised by theDutch mariners, when they wish to determine the altitude of the sun. Adecided improvement of this method was first indicated by Tarde, aFrenchman, who published a work on the solar spots in 1620. He statesthat when he observed the spots he placed a thick piece of blue or greenglass between his eye and the eye-glass of the telescope. By placing thecoloured glass in this position, the indistinctness of the image arising fromimpurities in the structure of the glass, or an imperfect parallelism of itssurfaces, was in a great degree removed.

The theory of the moon’s motion is a subject of so much importance,that the attention of astronomers is constantly directed to its improvement.We have already alluded to a method assigned by the theory of gravitationfor determining the distance of the moon from the earth. By means ofthe formulae of Laplace, Burchardt (assuming the moon’s mass to beof the earth’s) found that the constant part of the lunar parallax under theequator amounted to 57' 0". Damoiseau determined the moon’s mass tobe tjL-, the mass of the earth being represented by unity, and hence de-rived 57' 0"‘9 for the constant of parallax. Plana makes the mass -fa,and hence computes the parallax under the equator to be 57' 3"T.

A more obvious method of ascertaining the horizontal parallax of themoon is founded upon a comparison of her apparent positions as deter-mined simultaneously at different places on the surface of the earth. Oneof the principal objects of Lacaille’s voyage to the Cape of Good Hope was to make observations of the moon for this purpose. By comparinghis results with simultaneous observations made at different places inEurope , and assuming the ellipticity of the earth to be gig, he obtained57' 13"T for the constant part of the moon’s equatorial parallax. Thisis equivalent to a parallax of 57' 4"-6, if the ellipticity be supposed equalto -gjig. Lalande, by comparing Lacaille’s observations at the Cape of Good Hope with simultaneous observations made by himself at Berlin,fixed it at 57' 3"-7. Burg determined it to be 57' i" by a similar com-parison of Lacaille’s observations with corresponding observations madeat Greenwich. Henderson, during his residence at the Cape of Good Hope in the years 1832 and 1833, determined a great number of decli-nations of the moon, with the view of arriving at a more accurate value ofthis element. By a comparison of his own observations with others, madesimultaneously at Greenwich and Cambridge, he obtained 57' L'B for theconstant of the equatorial parallax j. This gives for the value of the

* Istoria et Dimostrazione intorno alle Macchie SoLri, p. 56.

f Mem. Ast. Soe.,vol. x., p.294.