204 



NATURE 



[DliCEMBER 31, I 89 I 



even for panoramas, but also furnished many valuable 

 hints with regard to light and shade, reflection and 

 chiaroscuro, and the general means of reproducing as 

 closely as possible on a level surface the raised appear- 

 ance of solid forms. A competent judge of both arts 

 might find it an interesting task to ascertain what share 

 photography has had in the origin of the modern schools 

 of painting, and in the manner of impressionists and 

 pleinairists. It further taught landscape-painters to 

 depict rocks and vegetation with geological and botanical 

 accuracy, and to represent glaciers, which hitherto had 

 been but rarely and never successfully attempted. It 

 caught and fixed the changing aspect of the clouds, 

 though only yielding a somewhat restricted survey of the 

 heavens. It aided portrait-painters without exciting their 

 jealousy ; for, unable to rival them in representing the 

 average aspect of persons, it only seized single, often 

 strained and weary expressions, rendering almost pro- 

 verbial the comparison between a bad portrait and a | 

 photographed face ; nevertheless it supplied them on j 

 many occasions with an invaluable groundwork, lacking 1 

 nothing but the animating touch of an artist's hand. | 



However, the recent progress of photographic portraiture I 

 claims the attention of artists in more than one respect. ! 

 Duchenne and Darwin called into existence a new 

 doctrine of the expression of the emotions ; the former j 

 by galvanizing the muscles of the face, in order to i 

 imitate different expressions, the latter by inquiring into j 

 their phylogenetic development in the animal series. 

 Both presented artists with photographs which quickly 

 consigned to oblivion the copies hitherto employed for 

 purposes of study in schools of art, dating chiefly from 

 Lebrun ; even the sketches in Signor Mantegazza's new 

 work on "Physiognomy and Mimics" will scarcely 

 enter into cou)petition. On Mr. Herbert Spencer's 

 suggestion, Mr. Francis Gallon subsequently solved by 

 the aid of photography a problem, which was previously 

 quite as inaccessible to painters as the representation of 

 an average expression to photographers. He combined 

 the average features of the face and skull of a sufficient 

 number of persons of the same age, sex, profession, 

 culture, or disposition to disease or vice, in one typical 

 portrait, which exhibits only those characteristic forms 

 common to their various dispositions. This was effected 

 by blending on one negative the faint images of a series 

 of persons belonging to the same description. In the 

 same manner, Prof. Bowditch, of Harvard Medical School, 

 Boston, obtained the representative face or type of Ameri- 

 can students of both sexes, and of tramway conductors and 

 drivers. In the latter instance, the intellectual superiority 

 of the conductors over the drivers is plainly visible. How 

 Lavater and Gall would have rehshed this ! 



Of course the average expression of a single person 

 might be procured by similar means, if it were worth ' 

 while summing up on the same plate repeated photo- 1 

 graphs of different expressions. Instantaneous photo- 

 graphy, however, furnishes a welcome substitute for the 

 average expression, by seizing with lightning swiftness 

 the changing phases of the human countenance in their 

 full vivacity. Here, again, pathology places itself at the 

 disposal of art. M. Charcot has found that photographs 

 of the convulsions and facial distortions of hysterical 

 patients resemble our classical representations of the 

 possessed. Raphael's realism in this respect is perhaps 

 the most curious of all, being so much at variance with 

 his idealistic nature. In the possessed boy of the 

 " Transfiguration," a cerebral disease can be almost safely 

 inferred from the Magendie position of the eyes ; and 

 the circumstance, recently observed in New York, that 

 the left hand is depicted in a spasm of athetosis, would 

 accord well with this diagnosis.^ 



{To be continued.) 



■ Sachs and Petersen, "A Study of Cerebral Palsies," &c., Jottmal of 

 Neri'ous and Mental Diseases, New York, May 1890. 



NO. I 157, VOL. 45] 



TELESCOPIC OBJECTIVES} 



TT is a frequent source of disappointment to observers, 

 ■*• especially beginners, to find that their instruments 

 fail to answer to the tests which are so commonly found 

 in astronomical text-books. It may be that the instrument 

 in question is really an imperfect one ; but if it be the 

 work of a maker of repute, it is more probable the fault 

 lies in the absence of proper adjustment, more especially 

 if, for some reason or other, no responsible person is able 

 to superintend the final fixing in position. The informa- 

 tion hitherto published on the subject of adjustment, and 

 the phenomena which accompany the various defects of 

 an objective, is very scanty ; and observers of all classes 

 will therefore welcome the appearance of the little book 

 recently issued by Messrs. T. Cooke and Sons, the well- 

 known firm of telescope makers The book is the best 

 testimony that one could wish for as to their thorough 

 knowledge of their business, and it abundantly demon- 

 strates that they are worthy of the confidence which 

 astronomers have long placed in them. The benefit of 

 their wide experience is now available to all, and observers 

 need no longer remain in doubt as to the quality of their 

 objectives, or of the course to be pursued in tracing the 

 defects to their proper sources. 



For full particulars of the methods to be adopted we 

 must refer our readers to the book itself, but many of the 

 points touched upon are of great interest, considered 

 simply as optical phenomena, and a brief reference to 

 some of them may not be out of place. 



It is a matter of common knowledge that, owing to the 

 undulatory nature of rays of light, the image of a luminous 

 point, such as a star, must always be a small disk, the 

 diameter of which varies in inverse proportion to the 

 aperture of the objective. This "spurious disk" is sur- 

 rounded by a series of diffraction rings, which gradually 

 diminish in intensity away from the centre. 



The calculations of Sir George Airy ^ show that the 

 angular radii of the rings, in circular measure, are given 



by the formula — , where X is the wave-length of the 



live 

 light-rays in question, e the radius of the objective, and 

 n a constant which depends on the distance from the 

 centre. The first dark ring occurs when n = 383, the 

 second when n = 7' 14, and the third when n = lo'iy. 

 Hence, the angular radius of the first dark ring, which is 

 really the boundary of the spurious disk, may be easily 



derived from the formula ^— ? — , or — . 

 lire 2e 



The rings are brightest when n — 5*12, 8"43, and ir63, 

 with intensities respectively about 1/57, 1/240, and 1/620 

 of that at the centre. 



If .f be the angular radius in seconds of arc, as viewed 

 from the centre of the objective, the formula becomes 



and if X for mean rays be taken as -000022 inch, 



n = I -3846 X es. 

 For the first dark ring, therefore, 



^^3-83 ^276 

 I -3846^ e 



Messrs. Cooke put these expressions in the form — 



Angulardiameterof firstdarkringincircularmeasure= , 



2F X I*22\ . 



a' ' 



Linear diameter of first dark ring = 



where A = aperture, and F = focal length. 



For a square aperture the conditions are different, and 



' " On the Adjustment and Testing of Telescopic Objectives." (T. Cooke 

 and S-^ns, Kuckingham Woiks, York.) 



^ "Undulatory Theory of Optics," 1877 edition, p. 80. 



