s6o 



NATURE 



[July 31, 19 15 



if not absolutely futile, much of the work done 

 by amateur theorists — namely, that observa- 

 tions for theoretical purposes must be made on 

 objects of a structure which is perfectly known 

 by some independent method, as otherwise we are 

 moving in a "vicious circle." It is clear that in 

 the case of the higher powers of the microscope 

 this difficulty cannot be avoided with delicately 

 marked natural objects examined directly. 



The numerical results obtained by Lister are 

 still of great value. 



For the naked eye he finds that there is practi- 

 cally no gain in resolving power when the pupil 

 is opened beyond C095 in. ; that the different 

 in keenness of vision of different individuals 

 rapidly decreases when smaller and smaller aperr 

 tures are placed before the eye, and that with 

 apertures below about 0025 in. all reasonably 

 normal eyes have the same resolving power which 

 corresponds to his general formula. In this 

 section we find a remarkable instance of the 

 accuracy of his observations ; for he records the 

 fact that — contrary to his preconceived idea — two 

 or three lines are more easily separated than a 

 larger number ; this has in recent times been 

 theoretically confirmed ! 



The telescopic section is the shortest, owing to 

 difficulties from unsteady air and bad light. But 

 it includes interesting measurements of the diffrac- 

 tion rings of the spurious star-disc. The limit of 

 resolution arrived at is 4'33 seconds of arc 

 divided by the diameter of the object-glass in 

 inches. 



Probably the most remarkable results are 

 those recorded in the microscopical section, which 

 also contains the most surprising proof of the 

 extraordinary accuracy of Lister's observations. 

 For these enabled him to deduce that the resolving 

 power of microscope objectives did not increase 

 in proportion to the angle of aperture, but to the 

 chord of that angle — in modern language, to what 

 Abbe, forty years later, called numerical aper- 

 ture. When it is considered that the widest 

 angle accessible to Lister was one of 8o°, and 

 that even for that the ratio of chord to angle 

 is merely that of 10: 13, it will be realised that 

 this was a very creditable feat. 



The limit of resolution for the microscope 

 arrived at by Lister, when translated into modern 

 terms, is 95,240 lines per inch for N.A. roo. It 

 should, however, be stated that although there 

 are a very few observations reaching or slightly 

 exceeding the figures finally adopted in the paper, 

 these are not the mean of all the observations. 

 Unlike some modern microscopists, Lister under- 

 stood the meaning of the word limit as a practi- 

 cally unsurmountable barrier which should only be 

 closelv approximated under extremely favourable 

 conditions, and he adopted his final numbers 

 accordingly. 



In conclusion I should like once more to advise 

 those interested in instrumental optics to read this 

 extremely interesting paper in extenso. 



A. E. CONRADY. 

 NO. 2283, VOL. 91] 



THE ANCIENT ARTISTS OF SOUTH- 

 WESTERN EUROPE. 



THE attention of readers of Nature has been 

 directed from time to time to the numerous 

 and interesting archaeological discoveries in the 

 caves and rock-shelters of central and southern 

 France and northern Spain, which are due to the 

 energy and skill of Prof. l'Abbe H. Breuil, MM. 

 L. Capitan, E. Cartailhac, Prof. H. Obermaier, 

 E. Piette, and others. During the last 

 two or three years similar investigations have 

 been made in other parts of Spain, mainly 

 by the indefatigable Abbe Breuil with the co- 

 operation of Juan Cabre Aguilo, Pascual Serrano 

 Gomez, and Gomez Moreno, and we should 

 like to take this opportunity of congratulating 

 the Spanish archaeologists on pursuing this 

 fascinating line of inquiry, which already has led 

 to important results. 



The epoch-making monograph "La Caverne 

 d'Altamira," by E. Cartailhac and H. Breuil 

 (1906), is being followed by the publication of 

 equally sumptuous memoirs on the more important 

 French caves. These, together with the numerous 

 papers and smaller memoirs that have already ap- 

 peared (mainly in V Anthropologic), prove that cen- 

 tral and southern France and Spain north of the 

 Cantabrian Mountains constituted an area through- 

 out which the later stages of Palaeolithic culture 

 were spread with remarkable uniformity. This 

 sequence consists of the Aurignacian, Solutrian, 

 Magdalenian and Azilian industries, with their 

 accompanying manifestations of glyptic and pic- 

 torial art. 



The earliest phases of the art of the Franco- 

 Cantabrian area are characterised by rude statu- 

 ettes of the human figure, which are followed by 

 beautifully executed carvings of animals in the 

 round or in relief, and by large numbers of en- 

 gravings on bone, ivory, and stone. The earliest 

 engravings are linear scrawls, and even rude 

 but vigorous drawings of animals on clay surfaces. 

 The walls of numerous caves were also decorated 

 with engravings and paintings of animals, of which 

 the following sequence has been established. 



First phase : the engravings consist of broad 

 deep incisions ; many of them are unrecognisable ; 

 some are profile representations of animals with 

 but two legs drawn; the paintings have a similar 

 character, but the earliest efforts were simple im- 

 pressions in colour of the human hand and timid 

 attempts at linear designs and grouped dots. 

 Second phase : the incisions are still broad and 

 deep, but the drawings are more lifelike ; the four 

 letjs are shown, and hair is often indicated; the 

 clever brush outlines of the earlier part of this 

 phase are replaced by monochromes, some of 

 which are beautifully shaded. Third phase : the 

 engravings are now in thin lines ; they vary in 

 excellence, but some are real masterpieces ; the 

 paintings are deplorable, owing to an excessive 

 use of pigment. Fourth phase : the engravings 

 lose their importance, and in some cases are merely 



