January 30, 191 3] 



NATURE 



607 



to form achromatic pairs. Tlie difference in the re- 

 fractive properties of fluorite and these glasses is not 

 less than in the ordinary glasses, such as enter into 

 the composition of the well-proved older achromatic 

 lenses, and it is sufficiently great to insure the flat 

 curvatures needed for spherical correction. The differ- 

 ence in dispersion is at least equal to that occurring 

 in the achromatic lenses, and hence no obstacles are 

 encountered in bringing two colours to a point. The 

 significant result derived from the use of fluorite is 

 that the difference of the quotients q reduces to 2-10 

 units, and almost disappears even in some combina- 

 tions, so that in addition to rays corresponding to the 

 D and F lines of the spectrum a ray of yet another 

 colour, corresponding to G', can be brought to a 

 point. The immense utility of fluorite lies in the fact 

 of its low refractive index being coupled with an 

 extraordinary small dispersion, by which it differs in 

 a striking degree from the glasses, whilst yet the 

 quotient is similar to that of the existing crown 

 glasses. The use of fluorite in the place of the usual 

 crown glass component rendered it possible to replace 

 the ordinary flint component with its disproportional 

 dispersion by a crown glass having either a quotient 

 agreeing with that of fluorite or at least differing but 

 slightly from it. 



The older silicate crown glasses, which had so far 

 been used in the composition of achromatic lenses, 

 contain a number of glasses which differ widely from 

 one another in their refractive and dispersional pro- 

 perties, whilst their quotient is much the same as 

 that of fluorite. By combining fluorite with tliese 

 crown glasses a means was obtained of producing 

 more perfectly achromatic lenses without the need 

 of a new glass. 



It will thus be seen that the new glasses were quite 

 a subordinate element in the composition of apo- 

 chromatic lenses. There is no doubt that their greater 

 range of variety has made it easier to produce apo- 

 chromatic lenses, but it is not essentially owing to 

 them that apochromatic lenses have come into e.xist- 

 ence. In 1891 Leitz made an attempt to produce 

 lenses of a higher degree of correction by the use 

 of glasses only. These were the so-called panta- 

 chromatic lenses, the optical qualities of which were 

 intermediate between those of the achromatic arid 

 apochromatic lenses. The attempt, however, had 

 soon to be discontinued since those glasses which had 

 proved the best means of endowing the panta- 

 chromatic lenses with a higher degree of colour cor- 

 rection proved to be liable to deterioration. Within 

 the last ten years several opticians have introduced a 

 new class of objectives, the so-called fluorite lenses, 

 in which the qualities of the former pantachromatic 

 lenses are realised with the aid of fluorite. 



These objectives have, so far as the author is 

 aware, the simple composition of the achromatic 

 lenses, and do away with the necessity of introducing 

 a triple lens, which renders them much less costly 

 than the apochromatic lenses. In their degree of 

 colour correction they approximate to apochromatic 

 lenses in proportion to the number of fluorite lenses 

 used in the system. Dispensing, however, with the 

 triple lens, they cannot be rendered equivalent to 

 apochromatic lenses, even when the number of fluorite 

 lenses is the same in both systems. On the other 

 hand, it is the presence of the triple lens which adds 

 materiallv to the cost of the apochromatic lenses. 



Reviewing the results achieved within the many 

 years during which the practical optician has been 

 guided and aided by the resources of science, it cannot 

 be said that any epoch-making progress has been 

 made. Yet it cannot be denied that modern men of 

 science and practical opticians have manifested an 

 extraordinarv activity in their keen desire to improve 



NO. 2257, VOL. 90] 



the power of the microscope and to extend our know- 

 ledge of the instrument. 



Comparing the performance of modern lenses with 

 those of thirty years ago one cannot fail to realise 

 that steady progress has been made. An objective 

 of numerical aperture i'4o is a remarkable piece of 

 work, and there is scarcely a modern lens that does 

 not bear testimony to the fruitfulness of recent efforts. 

 New types of objectives have likewise been devised for 

 the needs of the photographer, and various new 

 devices for observation by dark-ground illumination, 

 espscially Leitz 's dark-ground condenser, have de- 

 veloped this method of observation in a surprising 

 manner. The indefatigable activity of opticians as 

 well as physicists has elucidated the nature of the 

 problems relating to the limits within which it is 

 possible to improve the microscope, and has given 

 us a better insight into the modus operandi of the 

 instrument. It may suffice to remind the reader 

 of Abbe's theory of optical instruments. 



To what extent the study of microscopic optics has 

 occupied the minds of research workers is eloquently 

 borne out by the vast literature which during this 

 period deals with the microscope. Of journals devoted 

 to the study of the microscope we may mention the 

 Zeitschrift jiir wissenschaftliche Mikroskopic, the 

 Journal of the Royal Microscopical Society, and the 

 Journal of the Quekett Microscopical Club. The 

 microscope forms also the subject of extensive works, 

 amongst which one may mention those of Abbe, 

 Dippel, Lummer and Reiche, van Heurck, Wright, 

 Spitta, and Carpenter. A number of meritorious 

 works have been published which, whilst dealing with 

 optical matters in general, go far to further the 

 development of microscopical optics. Of the authors 

 who haVe written on geometrical optics and optical 

 instruments we mav mention Ferraris, Herman, Max- 

 well, Lord Rayleigh, Heath, Gleichen, Drude, Czap- 

 ski, von Rolir. Whittaker, Gullstrand, Leathem, 

 Schwarzschild, Maclaurin. 



It has often been said that the microscope has 

 reached the limits of its resources. Certain it is that 

 .it has needed the application of the utmost skill 

 and the most strenuous efforts to enhance the powers 

 of the microscope during recent years. There is, 

 however, every prospect that the ever-extending use 

 of the instrument, the increasing demands made upon 

 it, the intense scientific attention bestowed upon its 

 development, and the fine training of the modern 

 optician will not fail to maintain progress. 



C. Metz. 



SOME OF THE NEXT STEPS IN BOTANICAL 

 SCIENCE.'^ 



WHEN one who has worked long in any field of 

 science speaks before an audience such as this 

 he is expected to say something about the condition of 

 his branch of science when he began work with meagre 

 and poorlv adapted apparatus, to contrast it with its 

 greatly improved condition to-day, and to dwell with 

 pride upon the finely equipped laboratories with costly 

 apparatus, especially designed for particular experi- 

 ments, to be found by the twentieth-century scientific 

 student. 



In order that we may properly orient ourselves 

 with reference to the area covered by the science of 

 botany to-dav, we shall have to go back a few decades 

 to understand what additions have been made to its 

 territory during this period of expansion. 



Consider for a few minutes the botany of forty years 



1 From an .-iddres* delivered before the American Association for the 

 .Advancement of Science at Cleveland, Ohio, December, 1912, by the 

 retiring president, Prof. C. E. Bessey. 



