TRANSACTIONS OF THE SECTIONS. 9 



On tJie Invisible Part of tlie Moon^s Surface. 



By the Kev. T. W. Webb, A.M., F.R.A.S. 

 Assuming' tte con-ectness, or at least probability', of Hansen's assertion that tlie 

 centres of iigm-e and grayity of the Moon are not coincident, and that consequently 

 a difi'erent condition, both as to surface and atmosphere, might possibly obtain on 

 its remoter side, an attempt was made to inqiiire whether there is any evidence of 

 progressive change in proportion as v^-e recede from the centre of the visible hemi- 

 sphere ; and it was shown that though there are departures of more than one kind 

 towards the limb from the types of form prevailing in the centre of the disk, yet 

 those variations are not suihciently consistent to lead to any reliable inference ; and 

 that on the whole it is not possible, from what we see, to form any satisfactory 

 conclusion as to the condition of the invisible region. 



Light. 



On a New Form of !S])ectroscope , in u'Mch Direct Vision is obtained with a 

 Single Prism. By J. Beowning. 

 Some time since it was suggested to the author by Mr. Huggins that a du-ect- 

 vision spectroscope, more powerfid than Iloffhian's, would be a valuable addition 

 to the iusti-uments used for spectrum analysis. If made portable for travellers, it 

 could be used in the manner of a telescope for observing differences in the solar 

 spectrum at various elevations, for the spectra of flames, the absoi-ption-bands 

 produced hj different liqiuds ; and, above all, it would be most readily adaptable 

 to telescopes for examining the spectra of stars. "Whilst the author was engaged 

 on various contrivances having this end in view, Mr. A. Herschel showed him a 

 single prism he had contrived, which answered the pui-pose. It was of the form 

 that has been teiined 3 to 1 right-angled, from the hypothenuse being three times 

 as large as the base. These proportions are very simple and easy of execution. 

 In this prism, which was of crovm glass 2'.5 specific gravity, refraction occurs both 

 in the ray of light entering at the face perpendicidar to the short side, but near 

 the point, and also on its leaving the prism by the short face, the correction of the 

 inclination of the ray, so as to make it emerge in the same line as it enters, being 

 effected by its perfomiing trvo internal reflexions. In making this kind of prism 

 of very dense flint glass, the author had foimd the task more diflicult than he had 

 anticipated : the acute angle rec[uii-ed considerable modification, and it became veiy 

 difficult to keep the path of the ray within the prism. The best residts had been 

 obtained by throwing the ray to the left of the prism, and correcting this by cement- 

 ing a small crown-glass prism to the short face. The refractive angle, exercising a 

 contrary dispersion, need not be more than 5° — scarcely more than a tenth of that 

 employed by Hoffman in his constniction. The three sm-faces of this prism, being 

 in use, must be all equally ti-ue, or the definition suflers. After the residt of all the 

 experiments he had made, the author would wish to express his conviction that 

 whatever advantages maj^ be gained on the score of portability or convenience in 

 use, they were more than coimterbalanced by the inferior performance of these 

 arrangements, when compared "wath plain prisms of the best workmanship. In 

 connexion with this paper, the author added some remarks on the great difficulty 

 encountered in workmg plane surfaces on extra-dense glass prisms, and exhibited 

 two wonderfully delicate instruments for measxmng the irregularities. With the 

 first, inequalities of o-q J-to of ^^ mch. could be taken by direct reading, and of 

 go^oo of an inch by estimation ; with the second, inequalities of the xo owoo- of 

 an inch were discoverable. The common method is to take the reflexion of a 

 distant image through a telescope ; but the prisms Mr. Browning has producad 

 fully justify his choice of du-ect mechanical means for proving the perfection of 

 their surface-planes. 



