June 28, 1894] 



NA TURE 



201 



travellers were most severely affected at the highest camp 

 on Golden Throne, though even here, when completely 

 at rest, the inconvenience was comparatively slight. 

 The ascent from this camp — probably about 3000 feet, 

 without any exceptional difficulties — took %\ hours. In 

 such a district as the Pennine Alps this distance would 

 probably have been accomplished in half the time. It is 

 therefore obvious that the difficulties in reaching such a 

 point as K2 will be extremely great, though possibly not 

 insuperable. Among these the weather will be one of the 

 most serious. This, in the Alps, is frequently not good ; 

 in Baltistan, unless Mr. Conway's experience was 

 exceptional, it is habitually abominable. 



Much interesting information in regard to physical 

 geology can be gleaned from this volume. The 

 beautiful illustrations enable us to form a good idea of 

 the magnificent snow peaks ; the lower mountains, as a 

 rule, seem to be more precipitous and shattered, more 

 weird and desolate than in the .Alps (Fig. i). The 

 climatal conditions are probably favourable to rapid 

 denudation : mud avalanches are frequent, sometimes 

 on a gigantic scale ; the fans of debris also are enormous. 

 Mr. Conway's account of the alluvial deposits in 'he beds 

 of the valleys, especially on the route to Leh, are most 

 interesting (Fig. 3). These often make it difficult to 



Fig. 3. — Remains of .\lluv:um, Lama-yuru Valley 



discover the limits to what glaciers have extended. At 

 the present day the larger ice-streams seem to end at 

 10,000 to 11,000 feet above sea-level, but here, as in 

 other mountain regions, they were once much greater. 

 Mr. Conway mentions the occurrence of old moraines at 

 various elevations down to about 6000 feet, and we can- 

 not be certain that this is the lowest limit. 



The volume contains, in addition to a map, three hun- 

 dred illustrations, process-block reproductions of drawings 

 by Mr. McCormick, some from photographs, and some from 

 original sketches. They represent not only the scenery, 

 but also incidents of travel, and greatly enhance the 

 value of the bonk. It is worthy to take a place, for literary 

 and artistic excellence, with Mr. Whymper's "Travels 

 amongst the Great Andes of Ecuador," for it is a record 

 of an adventurous journey, carefully planned and bravely 

 effected, as well as a real contribution to knowledge. 

 Clearly and pleasantly written, full of interesting in- 

 formation, not only on the geography, geology, and 

 natural history, but also on the people, buildings, and 

 customs of a rarely visited region, the book does honour 

 to its genial and able author and to his companions in 

 travel. 



A NEW FORM OF OBJECT-GLASS MOUNTING, 



A MONG the numerous details in telescopic con- 

 ;^*- struction that have become of greater importance 

 in consequence of the increase in size of refractors, may 

 be mentioned that relating to the form of mounting of 

 the object-glass, which plays a leading role in the good 

 working of a telescope. 



The chief difficulty which has to be overcome is 

 the great dearth of substances which have the same 

 coefficient of expansion as glass. For small objectives 

 brass fastenings are used, but for larger discs these have 

 been replaced by the adoption of steel, the coefficient of 

 expansion of which more nearly approaches that of glass. 

 In the latter case allowances have to be made for the 

 I erence of expansions of the two substances (glass 

 an J steel), and this is done by breaking the metallic 

 band which encircles the objective, and connecting the 

 two ends thus freed by means of one or more screws. 

 In this way the pressure of the band on the circum- 

 ference of the discs can be regulated by tightening or 

 slackening the screws as the case may be, and the 

 inequality of expansion or contraction can thus be 

 counterbalanced. 



If only one disc of glass were in question, the problem 

 would be somewhat simple, but 

 since an objective consists of 

 two discs, and these of different 

 kinds, each possessing its own 

 special coefficient of expansion, 

 the matter is distinctly compli- 

 cated. With two discs it will 

 be seen at once that the metallic 

 ring may be made fast for one, 

 while the other can be free to 

 move, and therefore quite loose ;^ 

 this naturally raises numerous 

 disturbances in the centering 

 of the lenses. 



The functions of a perfect 

 cell are then, firstly, it must be 

 capable of holding the lens 

 firmly and without change of 

 form ; while, secondly, it must 

 be so arranged as to allow for 

 the different changes brought 

 about by temperature without 

 disturbing the centering of the 

 lenses. 



To produce such a cell as 

 this has been the obiect of 

 Dr. R. Steinheil's investigations ' and the following sum- 

 mary contains his suggestions. The principle con- 

 sists in leaving a space between the inside of the 

 cell and the circumference of the two lenses, and 

 placing rigidly between them blocks of particular sub- 

 stances and sizes, such that they compensate for the 

 different expansions at work. 



The amount a substance expands or contracts depends 

 not only on its increase or decrease in temperature, but 

 on its length ; thus, for instance, a long rod when heated 

 expands more than a short rod of the same substance. 

 Making use of this fact, we may either assume the Uiii^th 

 of the blocks referred to above, and calculate of what 

 substances it must be composed to give the exact co- 

 efficient of expansion required, or we may take any 

 substance with a knou'ii coefficient of expansion, such as 

 zinc, and determine the length it is required to be. The 

 latter method, of course, by its simplicity commends itself, 

 and if the length of the block be denoted by / we have 

 the formula > _ <^--'y 



/ = 



NO. 1287, VOL. 50] 



1 See "Ueber e!ne neiie Art von Objeklivfassungen " mZtituhrif! fur 

 InstritntcntenkutuiCf Hefts, P* 170, 1894. 



