Intelligence and Miscellaneous Articles. 75 



figuration of the land. He then described the wide distribution of 

 Snap-Fell granite, especially referring to its occurrence in radial 

 lines from the granitic mass, and called attention to the fact that 

 detritus of various rocks in the vicinity of the lakes has been carried 

 over the Stainmore ridge into the valley of the Tees. Mr. Curry 

 then described the occurrence of drift along the western slope of the 

 Pennine chain, and from Castle Carrock across the northern end of 

 that chain, as well as in the valleys of the Tyne and the Tees, 

 pointing out also the absence of drift from Alston Moor and Upper 

 Teesdale, and down the valley of the Wear to the city of Durham. 

 In conclusion the author discussed the manner in which the drift- 

 materials had been transported, referring it chiefly to marine opera- 

 tions on ancient shore-lines at various altitudes ; and in explanation 

 of the fact that the upper limit of the drift is not at a uniform ele- 

 vation, he suggested that it may in great measure be due to a varia- 

 tion in the volume of the ocean, instead of to elevations and depres- 

 sions of the earth's crust. 



XI. Intelligence and Miscellaneous Articles. 



OBSERVATIONS BY A NEW OPTICAL METHOD. 

 BY A. TOPLER. 



THE want in practical optics of some method of detecting before- 

 hand the existence in glass intended for lenses of places of vary- 

 ing densities, so-called tears (Schliere), led the author to construct an 

 apparatus, which not only achieves the object in question, but has also 

 been found applicable to determining differences of density in all 

 possible transparent media. 



The light proceeding from a bright lamp through the aperture of 

 a small shade falls upon a system of lenses of large aperture as apla- 

 natic and achromatic as possible (the head of a photographic appa- 

 ratus). The transmitted light concentrates in a focus at a distance of 

 from 10 to 25 feet on the other side. A simple Keplerian telescope 

 consisting of two lenses is so set up that its optical axis coincides 

 with the axis of the system of lenses in question, and that the focus 

 of the rays issuing from the s}rstem of lenses lies in front of the 

 object-glass of the telescope. The telescope, moreover, is so drawn 

 out that the rays issuing from the eyepiece have their point of junc- 

 tion in the pupil of the eye, and hence project on the retina the 

 image of a uniformly illuminated field of view. If a screen be 

 laterally pushed in front of the object-glass of the telescope, as soon 

 as its edge passes by the place of the focus the field of view will 

 at once become dark. If either in the glass of the system of lenses 

 in question, or in media which are interposed before or behind it, 

 there are places of different densities, some rays will thereby be de- 

 flected from their path, not pass through the focus, and hence not 

 disappear when the screen is pushed across ; these rays give then 

 in the dark field of view an image of the tear, by which name the 

 author designates in general places of different densities in a trans- 

 parent medium. 

 . The apparatus constructed by this method served, at first, only 



