1SG5.J Astronomy. 267 



image unaffected, but gradually extinguish the extraordinary image, it 

 is plain that those parts of this image which are of the least intensity 

 will be extinguished first. Thus as the prism rotates, the extraordinary 

 image of the spurious disc will gradually contract by extinction into 

 a mere point at the centre of the disc. It will be observed that all 

 depends upon the variation of the intensity of the light from the 

 centre to the circumference of the spurious disc, formed originally in 

 the focus of the object-glass. Nothing of the sort takes place with a 

 planet, inasmuch as the light does not sensibly vary in intensity from 

 the centre to the margin. 



At the January meeting of the Society, the Astronomer-Koyal 

 gave a comparison of the transit-instrument, in its ordinary or rever- 

 sible form, with the transit-instrument in its non-reversible form, as 

 adopted at Greenwich, the Cape of Good Hope, and other observatories. 

 In the use of the reversible transit the line of collimation is found by 

 observation of a fixed mark, or something equivalent to it, in reversed 

 position of the pivots; in the use of the non-reversible transit the 

 line of collimation is found by observation of two opposite collimators, 

 which have been adjusted one upon the other. In the determination 

 of errors of level and errors of azimuth there is no difference of 

 method requiring notice at present. Both systems are theoretically 

 quite accurate ; no constant error attaching to either of them if the 

 instrument is rigid, its pivots circular, and the bearings correct. The 

 points, then, which require consideration at present are not the great 

 and fundamental principles of construction, but the smaller points of 

 flexiu-e, irregularity of pivot form, and bad bearings. The Astronomer- 

 Royal's communication was occupied principally with the consideration 

 of the effects of flexure. It will be impossible to follow in abstract 

 the mathematical considerations respecting flexure of the axis, flexure 

 of the telescope tube, and the geometrical effect of such displacements 

 on the path of the optical axis of the telescope. The conclusion 

 arrived at being, that on every point the non-reversible transit-instru- 

 ment is superior to the reversible or ordinary transit-instrument as 

 commonly used. 



One of the most important contributions to Physical Astronomy 

 which has been laid before the Society for some time past, was com- 

 municated by Mr. Huggins at the same meeting. It had reference to 

 the much debated question of a lunar atmosphere. The author con- 

 sidered that some important information on the subject of a lunar 

 atmosphere might be furnished by observing the spectrum of a star a 

 little before, and at the moment of, its occultation by the dark limb of 

 the moon. It is well known that, from ordinary telescopic observation 

 of the disappearance of a star under these circumstances, no indica- 

 tions of a lunar atmosphere have been detected. From the absence of 

 such indications " we are," according to Sir John Herschel, " entitled 

 to conclude the non-existence of any atmosphere at the moon's edge 

 having one 1980th part of the density of the earth's atmosphere." 

 When, however, the observation is made upon the spectrum of a star 

 before, and at the moment of, its disappearance, several phenomena 

 characteristic of the passage of the star's light through an atmosphere 



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