1881.] 



Notes on Physical Geology. 



An 



the angle subtended at the slit by the objective of the collimator. It 

 should be a little larger than what is just sufficient to take in the 

 largest pencil that is to be observed, but not beyond that. The object 

 in keeping it as small as conveniently may be, is that only a trifling 

 change of focus may be required when the instrument is pushed aside 

 altogether, and the slit viewed directly through the spectroscope, 

 without the slight loss of light due to the two reflections. 



The compensating plate is represented as placed at the narrow end 

 of the prism, which permits of the two being cemented together, 

 thereby facilitating the support. I do not think that the minute 

 quantity of light which is reflected at L, and scattered at the surface 

 (even though blackened) FC in such a direction as to mingle with the 

 direct light would be any inconvenience, being too faint to be visible 

 at all. If it were wished to avoid this, or to get more easy access to 

 the surfaces AD, BC, for cleaning if requisite, the plate might be 

 placed at the other side ; but in that case it must not be cemented to 

 AB, as that surface is wanted for total reflection. 



The little instrument I have suggested may conveniently be called 

 a slit-rev erser, to distinguish it from other arrangements which have 

 been proposed, and in which the spectrum itself is reversed. 



P.S. Feb. 21. — The method proposed above is more directly appli- 

 cable to such an object as the comparison of really or apparently 

 coincident lines in the spectra of two elements than to astronomical 

 measurements, because in the latter case a great part of the difficulty 

 arises from a want of perfect accuracy in the clockwork movement of 

 the equatoreal. Yet I cannot help thinking that even for astronomical 

 work the method will be found useful ; for we can pass in a moment 

 from the direct to the reflected image of the slit, and vice versa, and 

 by taking the measures alternately in the two modes, and combining 

 them exactly as in weighing with a balance that is still swinging, any 

 error progressive with the time would tend to be eliminated, 



II. " Notes on Physical Geology. No. VII. On the Secular 

 Inequalities in Terrestrial Climates depending on the Peri- 

 helion Longitude and Eccentricity of the Earth's Orbit.'' 

 By the Rev. Samuel Haughton, Professor of Geology in 

 the University of Dublin. Received February 19, 1881. 



The attention of geologists was first called by M. Adhemar, and 

 afterwards more fully by Mr. James Croll, to the possible importance 

 of these long inequalities in climate, in explaining the climates of 

 geological periods, which differ considerably from those of the present 



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