May I, 1879] 



NATURE 



13 



any known gradation of sea temperature. It not only 



■ rcounts for the eocene heat when the land in the 70th 



.1 80th parallels was upheaved by enormous volcanic 



ion ; the cooler miocene brought about when this action 



jsided, and permitted Arctic seas to again find egress ; 

 ..i.J the cold glacial period when both shores of the 

 Atlantic were frozen by icy currents, as one shore is now ; 

 but by shutting off Antarctic currents it might have pro- 

 duced the hottest cretaceous times. Even the latter 

 ?u imposition is rendered hkely by the past and present 



tribution of life, and such conditions doubtless did exist 



remote times. 



I am, however, speculating beyond the scope of 



my present paper, for, however the eocene climate 



13 produced, the Arctic floras, I beheve, flourished 



it. Again I will close my paragraph with an extract 

 irom Dawson : ' " But overlying this plant-bearing 

 formation we have an oceanic limestone (the Niobrara) 

 .... indicating that the land of the lower cretaceous 

 ^vas replaced by a vast Mediterranean Sea, filled with 

 varm water from the equatorial currents, and not invaded 

 by cold waters from the north. This is succeeded by 

 thick upper cretaceous deposits. . . . these show that 

 further subsidence or denudation in the north had opened 

 a way for the Arctic currents, killing out the warm-water 

 animals of the Niobrara group, and filling up the Mediter- 

 ranean of that period." J. Starkie Gardner 



AN ENGLISH MICROSCOPE FOR THE USE 



OF STUDENTS OF MINERALOGY AAD 



PETROLOGY 

 T T may interest those who are studying petrology to 

 ^ know that a new microscope, specially suited for 

 mineralogical and petrological research, has recently 

 been constructed by Mr. T. W. Watson, of Pall Mall. 



For several years past students have frequently asked me 

 to recommend some microscope to them which would 

 answer their requirements, and, finding that none of the 

 cheaper instruments manufactured in this country were 

 supplied with concentrically-rotating stages, bearing 

 divided circles, and that even the high-class instruments 

 failed to fulfil all the requirements, it appeared that this 

 want might be supphed at a moderate cost, if one of our 

 instrument-makers could be induced to make a few trials. 



An examination of one of the microscopes devised by 

 Prof. Rosenbusch and manufactured by Fuess, of Berlin, 

 showed me that, although that instrument possessed 

 many features of great merit, it also had certain defects 

 which could be best overcome by adopting and modifying 

 a good English model. 



The great defects in most of the microscopes built on 

 the continental patterns consist in their fixed vertical 

 position, the smallness of their stages, and, very com- 

 monly, in the absence of any means of coarse adjustment, 

 except by a sliding movement of the body or tube, which, 

 if working stiffly, is very inconvenient, while, if sliding 

 easily, is apt to be shifted by a very slight touch. 



The microscope of Prof. Rosenbusch, apart from one 

 or two of these defects, is a very admirable instrument, 

 and presents various advantages over all other micro- 

 scopes hitherto made. 



The instrument, now manufactured by Mr. Watson, is 

 m most respects quite equal in performance to Rosen- 

 busch' s microscope, so far as the mechanical appliances 

 and adjustments are concerned, and is, I think, in point 

 of convenience, decidedly superior to the latter instru- 

 ment. 



The foot is a brass casting of a pattern somewhat simi- 

 las to that of Ross and other well-known makers. Upon 

 this a gun-metal limb is supported on trunnions, which 

 constitute the axis upon which the limb turns, so that the 

 instrument can be inclined at any angle, or placed in a 

 horizontal position for drawing. The right trunnion 



* The Princeton Rn'Irw^ 1879, p. 282 



carries a clamp to fix the instrument at any angle. The 

 lower portion of the limb bears the mirror, attached to a 

 jointed arm. The upper part of the limb is bowed, or 

 goose-necked, which renders it convenient as a handle, 

 by which to lift the stand, without entailing any strain 

 upon the working parts of the instrument. Above the 

 curve it is ploughed out to receive the rack of the body or 

 tube (on the pattern known as the "Jackson Model"), 

 and the coarse adjustment is effected by a pinion turned 

 by milled heads. The fine adjustment is of the usual 

 kind, and is situated near the lower extremity of the tube. 

 In the stand first made the milled head of the fine adjust- 

 ment was divided for the measurement of the thickness 

 of sections, but in future it is proposed to effect this object 

 in a different manner by divisions engraved upon the 

 limb and the sliding portion of the coarse adjustment. 



The head of the tube or body carries a bevelled disk 

 which is divided to 10° spaces. A corresponding disk 

 with an index is attached to the bottom of the analyser- 

 fitting, and rests directly upon the fixed divided disk ; so 

 that the analyser can be set in any required position, and 

 any amount of revolution imparted to it can also be 

 registered. The eye-piece, when inserted, is kept in a 

 fixed position by a stud, which falls into a small slot. 

 Crossed cobwebs are fixed within the eye-piece for the 

 purpose of centring the instrument. A small plate of 

 calc-spar, cut at right angles to the optical axis, is mounted 

 in a little metal ring, which can be placed between the 

 eye-glass and the analyser for stauroscopic examinations. 

 At the lower end of the microscope-tube a slot is cut to 

 receive a Klein's quartz plate or a quarter-undulation 

 plate, both of which are set in small brass mounts. 

 When these are not in use the apertiu-e can be closed by 

 means of a revolving collar. 



