362 



KNOWLEDGE. 



October, 1913. 



descriptions of some of the later designs of apparatus, 

 although they will deal with the theory of dark- 

 ground illumination in general. 



The most general methods in use are as follows : — 



(1) Siedentopf and Zsigmondy's Ultra-microscope, 



which, however, is inconvenient in biological 

 work, although it has been employed at 

 times. 



(2) Sub-stage Dark-ground Illuminators. 



The simplest type is the central stop in the 

 immersion condenser of high N.A., which, however, 

 is strongly chromatic. 



The simpler forms of dark-ground illuminators, 

 such as the paraboloid of Zeiss and reflecting con- 

 densers of most optical firms, are primarily designed 

 for fine dark-ground structural work such as the 

 observation of bacteria ; but, as mentioned above, 

 with a bright source of light they render visible 

 ultra-microscopic particles. 



The ultra-condenser of Leitz, designed by Dr. 

 Jentzsch, and the cardioid condenser of Zeiss, by 

 Dr. Siedentopf, are both primarily designed for ultra- 

 microscopic observations, but are not very suitable 

 for biological work on account of the delicacy of 

 adjustment required. 



In most of the dark-ground illuminators the rays 

 fall on the cover-slip at such an angle that total 

 reflection takes place, so that a clear field appears 

 black. When objects are present they shine out 

 brightly by the light which they scatter. 



In using immersion objectives, special means 

 must be taken to reduce the aperture — that is, to 

 cut out the peripheral rays by means of a diaphragm. 

 Apochromatic objectives always give the best 

 results, and the illuminators themselves are usually 

 achromatic. 



(3) By stopping the objective. 



This method was also devised by Dr. Siedentopf. 

 A certain circular area of the centre of the front 

 lens of the objective is blackened at the back. A 

 condenser of low N.A. is used and stopped down 

 till the direct cone of light is just blocked by the 

 blackened area. When objects are introduced into 

 the field they scatter the light. The method is 

 chiefly used for thick preparations, but has the 

 disadvantage of showing strong diffraction rings. 



With regard to the illuminant, in most cases the 

 better the light the better are the results. A Nernst 

 lamp gives fairly good results with the paraboloid, 

 for example, but more especially for the dark-ground 

 structural observations. A small arc lamp is better, 

 but best of all is strong sunlight directed on to a 

 large glass globe filled with water, although this light 

 cannot always be switched on when required ! 



Such is an indication of the methods employed. 

 A short list of literature will be given at the end. 

 The study of living material is usually most instruc- 

 tive, so that in most cases water has to be used as 

 the mounting fluid. 



A few of the applications of the method in 



botanical work will now be shortly described. 

 Little will be said of the work included in the first 

 two classes below, as many of the observations are 

 mostly of interest to the specialist alone. The main 

 lines of work may be perhaps classified as follows: — 



(a) The study of living bacteria, and so on. 



(6) The study of moving cilia. 



(c) Observations of the living plant cell. 



(a) Little will be said here. The method has 

 been useful in studying bacteria in the living state 

 generally, especially when one dimension is sub- 

 microscopic. The presence and movement of the 

 flagella have also been observed in the living state. 



(b) The method greatly facilitates the study of 

 cilia generally, as these are easier to observe as a 

 bright line against a dark background than in 

 direct illumination. 



Ulelah has recently published his results of a 

 series of researches by means of the method ; he 

 used, in fact, a Zeiss paraboloid. Motile cells from 

 practically all the great groups were examined — 

 zoospores of green and brown Algae, spermatozoids 

 of Bryophyta, and so on, as well as many Flagellata. 

 The results are, of course, to be appreciated by the 

 plant physiologist, but generally the work is of 

 interest in showing that the method is capable of 

 profitable application in this direction. 



(c) It is. however, with regard to the structure of 

 the plant cell that the most important results have 

 been obtained. 



The method of illumination employed, demon- 

 strates the presence of structures and particles 

 which cannot be observed in direct illumination, and 

 it also renders much more distinct some particles 

 which on account of their transparency and small 

 size are very difficult of observation in direct 

 illumination. 



The greatest difficulty to be encountered in the 

 application of the method to the study of the plant 

 cell is that of choosing and obtaining suitable 

 material which can be examined in the living state. 

 The object must, if possible, be only one cell thick, 

 which at once precludes the use of masses of tissue, 

 or sections of tissue generally. The cell walls must 

 be optically homogeneous, or nearly so, in order to 

 allow of an illumination of the cell contents, and, 

 moreover, the outer wall of the cell must be quite 

 clean. When all these desiderata have been 

 obtained the cell contents may be quite unsuitable 

 — for example, a large peripheral chloroplast scatters 

 most of the light at once, so that no internal cell 

 structure can be seen. 



Objects which may furnish suitable material 

 are more especially unicellular organisms, such as 

 unicellular algae, yeasts and so on, filamentous Algae 

 and Fungi — Spirogyra, Mttcor, flat plates of cells — 

 tnonostroma, thin leaves, unicellular or seriate plant 

 hairs, root-hairs, and so on. There is still another 

 point which rapidly becomes evident when observa- 

 tion is begun — the cells must not be too small nor the 



