April 1, 1901.1 



KNOWLEDGE. 



on 



of bones of olepliants and hippopotami. This remarkable 

 fanna existed al a time when tlio JNIaltese area formed a 

 portion of the great Mediterranean land bridge, and 

 when the elimatic conditions of southern Kuroiio were 

 very different to those that now endure. 



The present rainfall of the iMalt.ese! islands 

 averages 17 inches, and it rarely exceeds 19 inches. 

 The amount of water, therefore, that annually passes 

 down the gorges to the sea has little or no erosive power, 

 and is barely sutBcient to remove the thin integument 

 of soil which covei-s the bottoms in the higher reaches. 



The question of the age of the Har Dalam cavern ancf 

 its fossilifcrous contents opens up an interesting field for 

 investigation. The depth of the gorge is about seventy 

 feet, and the cavern is situated at a height of forty feet 

 from the bottom. The limited hydrographical area of 

 the gorge renders it impossible, whatever the rainfall 

 might be, to fill the gorge to a height sufficient to fill 

 the cavern. The upper series of floor deposits were 

 therefore laid down at a time when the gorge was very 

 much shallower than it is now. It is hardly possible to 

 express the antiquity of these layers in terms of written 

 history, for the estimation of time by the rates at which 

 rocks are denuded and built up is dependent upon so 

 many variable factors that such chronometers are at best 

 uncertain and unreliable. The Phoenician temples, 

 tombs, and water tanks which are found in this gorge, 

 at a level lower than that of the Har Dalam cavern, 

 indicate how little the forces of erosion have effected 

 since the time when they were constructed, about- 3000 

 years ago. 



Recent researches in the Nile delta have shown that 

 two separate cultures existed prior to the advent of the 

 Egyptian dynasties and Pyramid builders, thus giving a 

 continuous history of 8000 years for man in Egypt. The 

 ruder stages that characterize the Paleolithic and Neo- 

 lithic periods in Europe are wanting around the Nile, 

 but this is explained by the fact that the alluvial deposits 

 of the Nile basin are not much more than 8000 years old. 

 Observations have shown that during the last 8000 years 

 the rate of deposition of Nile alluvium has averaged 

 about one yard in a thousand years. 



The borings indicate that the average maximum thick- 

 ness of the Nile mud in the valleys is about eight yards, 

 and this points to the conclusion that immediately prior 

 to the time when the first layer of mud was formed, the 

 rainfall was sufficient to fill the river valleys and prevent 

 the "dejDosition of the alluvium. The configuration and 

 general physical features of the Egyptian Uieds and the 

 Maltese gorges have much in common. The elimatic condi 

 tions of the two areas are now, and probably always have 

 been, similar. It is, therefore, probable that the 

 work of gorge erosion was contemporary. and 

 that the depth and extent of the Har Dalam 

 gorge difiter but little from what they were at 

 the time when the great change in the climate 

 set in. If it were possible to fix the exact time when 

 the upper series of beds in this cavern was deposited, 

 then the age of the stone implements and other relics 

 of human industry which they contained would be deter- 

 minable. It is not probable that this will ever be done. 

 To do so it would be necessary to revert to a period when 

 the streams that coursed down the gorge were of suffi- 

 cient magnitude and power to rasp away the gi-eater 

 portion of the forty feet of hard semi-crystalline limestone 

 that once lay between the mouth of the cavern and the 

 present bed of the gorge. The time occupied by the 

 waters of the stream in the work of corrosion and erosion 



must be added, and this, as li.is .ilrcadv been pointed out. 

 is an indeterminable factoi'. 



The evidences aft'ordetl by llic Siciliiiu caves and I lii' 

 ossiferous loams of (he JIar Ualam cavern bring the 

 solutions of the problem bearing on the migi-ation.s of 

 ]ire-historic man in the central Mediterranean one step 

 nearer. It is true the step is a short one, but the work 

 of exploration proceeds apace, and the time will come 

 when the gulf wiiich at present separates the domain of 

 the antiquarian from that of the geologist will be bridged 

 by discoveries that will clear away the mist of un- 

 certainty with which the subject is at present obscured. 



Conducted by M, I. Cross. 



SuHST.MiK (JiNnr.NSKKS. — II is gratifying to observe the 

 number of first-class substage conden.sers that are offered by 

 manvifacturers, and it is a distinct indication of growing know- 

 ledge and appreciation of good things on the part of workers. 



It was at one time an easy matter to make a choice when only 

 two or three systems were available, but it is evidently present- 

 ing some complexity now, and in response to correspondents 

 enquiries we pro])Ose to give a few bints on the subject. 



The main features of a condenser are : (1) 7'//*? /irhroniallxiii, 

 (2) (iphdKiliiini, (?,) iintr/n/fi/i>i</ potuer, and (4) Ihe. .sv'.-t? nf llie 

 Jixed teii.i. 



Aclirniiiatiitiii and iiplii.nnlixm can be considered together, but 

 the latter is the more important. Recognising this, there is a 

 tendency on the part of makers to claim greater aplanati.sm 

 than is actually yielded ; this can, however, easily be verified by 

 the methods described in the text-books. Achromatism is a 

 desirable quality, but we doubt the advantage of an apocbro- 

 matic over an achromatic condenser ; we would as readily work 

 with the latter as the former ]>rovided the aplanalism were as 

 well corrected, and this is freciuently the case. I^^xpcnse m.ay 

 therefore be avoided without loss of efficiency in this respect. 

 The solid illuminating cone that an objective will bear has been 

 frequently discussed. It is generally stated that three-fourths 

 the full ai)erture is the best, but it will be found that the 

 majority of lenses will not bear more than two-thirds without 

 deteriorating in performance ; there are some exceptional ones 

 that will take more than a three-quarter coue, but this is not the 

 rule, and a light filter is usually requii-ite. 



The potrpi-. — The magnifying ])Ower of the condenser should 

 not exceed half that of the objective, less rather than more than 

 half is always preferable. Many sy.stcms are .arranged to work 

 satisfactorily with the front lens removed, and by this means 

 high and low power effects are secured in one combination. 



S>::e nf field leii-^. — The reason for the popularity of the Abbe 

 illuminator, with its glaring imperfections, is on account of its 

 large field lens and the ease with which it can be worked. A 

 high power condenser must of necessity have comparatively 

 small lenses, and requires as great care in manipulating as the 

 objective itself. The Abbe achromatic condenser was an 

 attempt to maintain the easy working of the Abbe illutninator 

 in a corrected form, but it is really too heavy and clumsy and 

 restricts the movements of a met-hanical stage. The best con- 

 den.sers have, as a rule, the largest field len.'es that can be 

 advantageously fitted, but this point is deserving of special 

 consideration when making a decision. 



Rerniamrnduiionn. — From the foregoing it will be possible, 

 with given objectives and a maker's catalogue, to choose the 

 most suitable condensers. If a man proposes to restrict himself 

 to low and medium powers, not exceeding say J in., he can 

 readily make a choice, and we would like to specifically mention 



