A. MATHEMATICS AND ASTRONOMY. 53 



to the telescope is so placed that at the point on the solar 

 disk where the contact is expected we shall behold the bright 

 lines of the sun's atmosphere, which extend, as is well known, 

 to a considerable distance from the glowing body of that lu- 

 minary. It is, therefore, apparent that when the planet ap- 

 proaches the solar disk, it Avill, first of all, cut off the bright 

 spectroscopic lines representing the solar atmosphere, and 

 in this w^ay will itself, as it were, become visible ; whereas 

 otherwise it would be invisible until it had entered upon the 

 bright solar limb. By this means the observer becomes per- 

 fectly prepared for the observation of the exact moment of 

 the contact of the dark disk of Venus and the bright disk of 

 the sun. This method has, indeed, been employed in. obser- 

 vations of solar eclipses, and wuth such success that it is 

 known that the moment of contact can thus be observed more 

 exactly than in any other way. 



Secchi has announced a modification of this method, and 

 has tested it in the instance of the j^artial solar eclipse that 

 occurred in Italy on the 26th of May last. His arrangement 

 is as follows : In front of the slit of the ordinary sj^ectroscope, 

 and about one inch distant therefrom, he places a dispersing 

 prism of considerable power. This prism offers, in the plane 

 of the slit, an image of the sun constructed of many-colored 

 rings, like a very impure spectrum. The rays that penetrate 

 from this image through the spectroscope give, in the field 

 of view of the small telescope, a very sharj) and clear image 

 of the sun, in which not only the limb, but also the dark and 

 bright spots are clearly seen ; in short, one sees the sun as 

 through a colored glass. The chromosphere, seen by a prop- 

 er arrangement of the apparatus, has a bright line, somewhat 

 distant from the sun's disk, corresponding to the altitude of 

 the chromosphere, and the spectral line tliat we choose to 

 employ. It follows, therefore, that the opaque planet will 

 cover the chromosphere on the corresponding point in the 

 field of view of the telescope ; the chromospheric line will be 

 broken ; and the observer thus is forewarned that the planet 

 approaches its contact with the sun. In making his first 

 trial upon the eclipse, Secchi w^as able to detect the approacli- 

 inc: and otherwise invisible moon eleven seconds before its 

 first contact with the sun's limb. In applying the same 

 method to the transit of Venus, on account of the slow mo- 



