I >ECEMBER 2, 1922] 



NA TURE 



739 



and 43 per cent, respectively. Several workers, and 

 especially the Hirschfelds, have shown that as one 

 travels from west to east the prevalence of Group II. (A) 

 decreases and that of Group III. (B) progressively 

 rises. In Western Europe, A is found in about 45 per 

 cent., in Russians and Arabs in 37 per cent., in negroes 

 and Indians in 27 per cent. B, on the other hand, 

 increases from about 15 per cent, in France, through 

 the Balkans (20 per cent.), Malagasies (28 per cent.), 

 negroes (34 per cent.) to Indians with 49 per cent. We 

 have here an obvious suggestion of two original races 

 of mankind, which have mingled in various degrees : 

 it is possible that in some remote place a pure A or B 

 variety still exists. 

 At present there is no evidence that these blood 



characteristics are associated with any other qualities, 

 and it seems likely, like some other Mendelian characters, 

 that they are negligible in the problems of selection 

 and survival. It would, too, be an error of the ancients 

 to suppose that the qualities of the blood dominated 

 personality and conferred a general characteristic on 

 the individual. There is much evidence of the essential 

 similarity of parents and offspring. The greater 

 success of grafting tissues from one animal to another 

 if they are of the same family is a germane example. 

 In blood tests brothers and sisters by no means 

 always agree so far as the agglutination of their cor- 

 puscles is concerned: in other respects their bloods 

 are probably more similar than those of more remote 

 relations. 



The History of the 



DR. REGINALD S. CLAY performed a needed and 

 useful service when he selected for the subject 

 of the twenty-fifth annual Traill-Taylor Memorial 

 Lecture, which he delivered at the meeting of the Royal 

 Photographic Society on October 10 last, " The Photo- 

 graphic Lens from the Historical Point of View." It was 

 a needed service, because a historical review of the origin 

 and development of the photographic lens is necessary 

 for a just estimate and balanced perspective of the 

 many and diverse scientific factors that have to be 

 taken into account in the production of modern photo- 

 graphic lenses. It was a useful service, because the 

 fascinating and, at times, almost dramatic 'story that 

 Dr. Clay had to tell brings out clearly the paramount 

 importance of the pioneer work done in this field by 

 British firms and scientific workers, and it must act as 

 a useful corrective, to the tendency, sometimes mani- 

 fested in unexpected quarters, to underrate the value 

 of British work in the optical field. 



After touching lightly on the early history, Dr. Clay 

 comes to " one of the great landmarks in the history 

 of optics — the invention of the achromatic lens." 

 John Dolland, after numerous experiments, exhibited 

 to the Royal Society an achromatic prism in 1758 of 

 crown and flint glass, and explained its construction. 

 Of the authors who contributed, in this period, before 

 the invention of photography, to the theoretical treat- 

 ment of the lens, Dr. Clay instances, after Kepler, the 

 following : 



Huvgens, who, besides expounding the wave theory 

 of light and the explanation of double refraction, also 

 dealt with the spherical aberration of lenses, and 

 showed how it varied with their aperture and focal 

 length ; Newton, who investigated the dispersion of 

 light ; Joseph Harris, who discussed the cardinal 

 points, optical centre, oblique pencils, curvature 

 of field, etc., in his " Treatise of Optics " ; Herschel, 

 who obtained valuable equations for the calculation 

 of objectives free from chromatic and spherical 

 aberration ; George Biddell Airy, who investigated 

 the conditions for eliminating astigmatism and 

 distortion ; William Hamilton, who evolved powerful 

 mathematical methods which even yet have not been 

 fully utilised ; and, last but not least, Henry Cod- 

 dington, who worked out the methods which, I 

 believe, still form one of the most useful bases for 

 attacking new problems in lens construction. 



The next milestone marks the almost simultaneous 

 announcements of the inventions of photography by 



NO. 2770, VOL. I 10] 



Photographic Lens. 



Daguerre in 1838 and Fox Talbot on January 30, 1839, 

 and we reach " the epoch from which we may date the 

 great evolution of the photographic lens." After 

 referring to the photographic lenses of Charles L. 

 Chevalier, Dr. Clay comes to the work of Josef Max 

 Petzval (1807-1891), who computed a new and most 

 successful lens, corrected for spherical aberration over 

 a small angular field, which was made by Frederick 

 Voigtlander in 1840. 



We may pass over much interesting record and come 

 to a new chapter, opened in 1866 with the aplanatic 

 lenses of Steinheil and Dallmeyer. Steinheil, " be- 

 ginning to recognise the value of symmetry in reducing 

 astigmatism and distortion," concluded that the 

 astigmatism would be less if the refractive indices of 

 the glass were more nearly equal ; he therefore used 

 two flints instead of flint and crown, putting the higher 

 refractive glass outside. Dallmeyer also used two 

 flints, and called his first lens a " wide-angle recti- 

 linear lens," 1866. It worked at f/15, and he followed 

 it by his symmetrical at f/7 and f/8. In 1874 Steinheil 

 made a portrait lens of two cemented lenses working 

 at f/3 - 5, and in the same year Ross brought out their 

 portable and rapid symmetrical, calculated by F. H. 

 Wenham. " This is of interest," says Dr. Clay, " as 

 Ross and Co. (as the firm then was) was thus the first 

 firm to employ a scientific man as calculator. Wenham 

 was with them from 1870 till 1888." 



The next step, which Dr. Clay describes as " the 

 greatest step in the development of the photographic 

 lens," was made possible by the new glasses — the barium 

 crowns of the Schott glass factory at Jena. The 

 problem and its solution is thus expressed : 



An achromatic lens of ordinary crown and flint, 

 which we may call an " old achromat," could be 

 corrected spherically, but not made anastigmatic. 

 An achromatic lens made of the new barium crown 

 and a flint could be corrected for astigmatism, but 

 not spherically. To correct both, all three glasses 

 must be used — old crown, flint, new barium crown. 

 To take full advantage of this principle, it is obvious 

 that each component can be made of all three glasses. 

 It can then be achromatic, anastigmatic, and 

 aplanatic. By combining two such components 

 into a symmetrical lens, it can also be made ortho- 

 scopic, and can easily be given a flat field. This is 

 the principle underlying the well-known Goerz lenses. 

 Another way to achieve the result is to use two unlike 

 combinations, one of which is made responsible for 



