HARDWICKE'S SCIENCE- GOSSIP. 



107 



the task of years. Except land-plants, no fossils 

 have been discovered iu the formation nearer than 

 Bridge of Allan. Further north, as everybody 

 knows from the writings of the late Hugh Miller 

 and others, the remains of a wonderful series of 

 armour-plated fishes are abundantly present. The 

 grey flagstones, containing] the plant-remains, are 

 best seen in the quarries in Cameron "Wood, near 

 the foot of Loch Lomond. The lake steamer from 

 Balloch to Balmaha takes the passenger close by 

 the islands of Inch Murriu, Torrinch, ^and Inch 

 Cailloch, giving him a good view of the great con- 

 glomerate near the base of the old red sandstone. 

 This conglomerate may be more closely inspected at 

 the pass of Balmaha, and may be followed inland 

 in a straight line, as its beds crop out nearly on 

 end, through Aberfoyle to the lower end of Loch 

 Venachar. In the Menteiih Hills it forms a very 

 striking feature of the landscape. Volcanic rocks 

 of this age occur in the Ochils, and in the neigh- 

 bourhood of Crieff and Comrie.* 



The Scottish old red sandstone is now generally 

 admitted to be a fresh-water formation, contem- 

 poraneous with the Devonian marine formation of 

 England and Belgium. Along a line from Balmaha 

 to Stonehaven, on the east coast, a fault whose 

 downthrow must amount to four or five miles, 

 separates the old red sandstone from the " primary 

 rocks" of the early geologists. 



We have now reached a minimum distance of 20 

 miles from Glasgow, and have passed over the 

 edges of strata whose total thickness is greater 

 than would be believed, and which, therefore, we 

 refrain from mentioning. The Highlands are 

 mainly composed of the so-called "primary" 

 rocks, whose structure is a subject much too wide 

 for the present sketch. Suffice it to say that they 

 are older than the old red sandstone, have been 

 crumpled into folds, metamorphosed, upheaved, 

 denuded, and to a large extent covered over with 

 newer, but still enormously ancient accumulations. 

 Within the last few years they have been recog- 

 nized through Mr. C. W. Peach's discovery of 

 fossils, as belonging, for the most part, to the 

 Silurian age, some Cambrian and Laurentian rocks 

 underlying the Silurian in the far North-western 

 Highlands. A general idea of the Silurian area may 

 be gathered in the course of a sail up Loch Long 

 and down Loch Lomond, or at Dunoon or Arran. 



It only remains to be added that a geological 

 excursion should never be undertaken without an 

 accurate map. It is astonishing how easily the 

 grand operations] of nature may be misinter- 

 preted for want of this simple corrective. The 



* A report of a lecture, by Prof. Geikie, to the Glasgow 

 Geological Society, published in Nature, vol. xiii. p. 389, will 

 enable the reader to appreciate the importance of the old red 

 sandstone as a sedimentary formation. 



contoured maps of the Ordnance Survey * are ob- 

 tainable at a trifling cost, contain a fund of infor- 

 mation regarding roads, levels, &c, and render the 

 traveller independent of the " intelligent native." 

 Putting one's geological observations on accurate 

 maps is excellent practice, and the habit, if culti- 

 vated, would 



" From many a blunder free us, 

 And foolish notion." 



THE MICROSCOPE AND MICROSCOPIC 

 WORK. 



No. IX -By P. Kitton. 



"IT^ITH the close of the last century the Micro- 

 scope seems in some measure to have fallen 

 into disuse. Very few works, either on its con- 

 struction or on the work done by it, seem to have 

 been published. Opticians were unable to produce 

 any better instruments, so far as the optical part 

 was concerned, than those used by Leeuwenhoek, 

 Baker, and others, whose labours we have been 

 reviewing. Attempts were made by M. Charles, 

 of the Institute, to construct a chromatic objective, 

 but these attempts were unsuccessful. Iu 1S12 Sir 

 David Brewster proposed to make both single and 

 compound microscopes achromatic in the following 

 manner : A drop of fluid of greater dispersive power 

 than the convex lens which formed the objective 

 was placed on the object about to be examined. 

 The lens was then immersed in the fluid, thus 

 forming a plain concave lens below the convex one. 

 This plan improved the performance both of double 

 and single instruments ; but, as may be imagined, 

 this method was very troublesome. He afterwards 

 contrived a more permanent form of achromatic 

 object-glass by placing some " butter of antimony " 

 (sesqui-chloride of antimony) between a meniscus 

 and a plano-convex lens. This was capable of being 

 renewed when necessary. 



The grand desideratum was, however, a compound 

 objective which should be aplanatic, and Professor 

 Amici, of Modena, made many experiments to 

 improve the achromatic objective, but with very 

 moderate success. He therefore turned his atten- 

 tion to the reflecting microscope, and with this he 

 was so successful that he gave up the refracting 

 instrument for several years. 



With these instruments many test-objects were 

 resolved, but the loss of light from so many reflec- 

 tions must have been very considerable. The follow- 

 ing diagrams will enable the reader to understand 

 the structure of the reflecting microscope. Pig. 109 

 represents Amici's plan: a is a concave speculum, 

 upon which the image of the object, c, is reflected 



* For Glasgow, Sheets 22, 23, 30, and 31, on the scale of 

 1 inch to the mile, are sufficient. 



