October, 1910. 



KNOWLEDGE. 



4i: 



In some cases, however, it may be necessary to increase 

 the contrast between the strnctnre and its surronndings. 

 It is not \-ery essential to make permanent preparations, 

 bnt the cedar oil specimens will last indefinitely. Fine 

 results are obtained by the use of the monochromatic system, 

 using ultra-violet rays ; Dr. KiJhler's figure of the blacU 

 scale of Pieris brassicac I ZeUschr. f. wiss. MikrosJi. Bd. 

 XXI, Taff. II) is the finest photograph of the dots and striae 

 that I have yet seen. But with an ordinary microscope the 

 facts may be made out, though it may not be possible to 

 produce such brilliant prints. 



Figures 1 to 5 were all taken with the 5 Projection Ocular, 

 and at the same position of the camera. 



ON DRAWING MICROSCOPICAL OBJECTS.— The 

 next best thing to a good description of an object, is a good 

 figure of the object described. It prevents many misunder- 

 standings ; it saves time in identification ; it makes the subject 

 more interesting to the student, and it often causes the casual 

 reader in Natural History subjects to pursue the study ftuther 

 than he would otherwise do. 



Many objects can now be figured by means of Photography, 

 and are largely so used in thejoiu'nals for the purpose of 

 illustration; but there are some objects, requiring the Micro- 

 scope to study or see properly, which do not lend themselves 

 to reproduction by means of Photography : they are too 

 thick, and under the Microscope all the points cannot be 

 brought into focus at the same time in consequence much 

 detail is lost. In these cases a drawing is better than a 

 photograph. While making a drawing one can gradually 

 focus down until all details are shown as required. Or 

 the stage of the microscope can be moved to bring parts 

 of the object into view which previously had been outside 

 the field. Manj- microscopists have an idea that it is 

 \'ery difficult to make a drawing of an object as seen under 

 the microscope ; certainly some objects are more difficult than 

 others, but anyone following the simple directions here given 

 cannot fail to make a presentable figure of any object required. 



There are several cameras in use for drawing with the micro- 

 scope ; the one used by the writer is Ashe's Camera-lucida, 

 a description of which, and how to use it, is given by Mr. 

 D. J. Scourfield. in the Journal of the Oiiekett Microscopical 

 Club (vol. vii, p. 413, 19001. It can be bought at most of the 

 leading dealers in microscopical instruments in London. Its 

 advantage over the old camera-lucida is that the image is not 

 reversed, but can be seen on the paper in the same positions 

 as seen under the microscope ; also that the microscope can 

 be left at an angle of say 45° while the drawing is being made, 

 thus doing away with the alteration of the position of the 

 microscope every time it is used with or without the camera. 

 Before commencing, it is necessary to make a few experiments 

 and calculations with the objectives in use by placing an 

 object of known size on the stage ; this enables one to 

 give the size of the object and that of the drawing in their 

 true relative proportions ; also, when once the magnification 

 is known, to measure up all the detailed work in the 

 drawing. It also enables one to decide which objective 

 to use and how long the draw-tube should be to produce 

 a drawing to a given scale of magnification. B\' working 

 all this out beforehand much time and trouble is saved. 

 The light through the object and the amount of light on the 

 paper will require nice adjustment, but when this is done 

 satisfactorily and one can see the point of the pencil on the 

 lines of the object, the drawing may be proceeded with, using 

 a hard lead pencil to put in the required lines. These pencilled 

 lines must then be worked over in Indian ink, using a fine 

 etching pen for the purpose ; when this is complete the pencil 

 drawing may be cleaned oft", leaving the paper with a pen and 

 ink drawing which can now be tinted and shaded up with 

 Indian ink or sepia to the required point of finish. The 

 advantages of the Indian ink being that a wash of colour can 

 be put on without any fear of blurring or disturbing the draw- 

 ing. Should the drawing require to be finished in colour, 

 make use of coloured waterproof inks as near the colours of 

 the object as possible, and tint and shade with ordinary water 



colours. There is not much art or practice required ; it is 

 simply careful work. The writer is bound to admit that the 

 waterproof inks of various colours are a real boon to the 

 microscopist who wishes to make drawings of his objects in 

 colour. These inks are made and supplied by all artists' 

 colourmen. and they can be mixed to form certain tints at the 

 time of using. Very often the inks are too strong in colour 

 for the purpose required and need diluting ; this can always 

 be done by using distilled water to which a drop of liquid 

 ammonia has been added. If ordinary water be used to 

 dilute the inks the colour will not stand washing over. 



Chas. D. Soar. F.R.M.S. 



PHOTOGRAPHY. 



By C. E. Kenxeth Mees, D.Sc. iLond.), F.C.S. 



THE USE OF THE PHOTOGRAPHIC PLATE FOR 

 RECORDING AND MEASURING INTENSITIES.— The 

 use of the photographic plate for the recording of position 

 with a view to subsec|uent measurement has been so general 

 that, in many branches of science, it has entirely superseded 

 direct visual observation. A striking example is supplied by 

 the almost complete disappearance of spectroscopes in favour 

 of spectrographs for all purposes of investigation, though here 

 the new interference methods of wavelength determination 

 are. to some extent, again displacing photography. But the 

 use of the photographic plate for recording the intensity of 

 the light falling upon it is by no means so general, and 1 his 

 for the very good reason that the interpretation of the results 

 is difficult and that, until recently, the relations between the 

 deposit produced and the intensity of the radiation producing 

 it, were somewhat obscure. The use of photography in this 

 way is, however, quite possible if adeipiate precautions be 

 taken, and, in view of the difficulties involved in all other 

 methods of measuring radiation, and especially such radiation 

 as violet or ultraviolet light, it would seem that for some 

 purposes the photographic method should be of considerable 

 value. 



The deposit on a developed and fixed plate consists of 

 almost pure silver. The mass of silver per unit area of the 

 plate can be foimd by measuring the transparency which it 

 shows for light, and Hurter and Driffield in their original 

 paper on the relations between the mass of silver produced 

 and the exposure and development of the plate, established a 

 useful nomenclature which has been generally accepted. 



If I be the light incident on the negative and Ii the light 

 transmitted then — 



I 



I 1 



— or -z^r- 

 h T 



The "extinction coefficient " will therefore be the logarithm 

 of the Opacity or log. ml - log. loL and this they termed the 

 Density (Dl. 



The Density of a plate' is therefore (from Beer's law) 

 directly proportional to the mass of silver in the plate. 



The " photometric constant '■ or mass of siher in grams per 

 square centimetre which corresponds to a Density of unity, 

 that is, to a plate which transmits one-tenth of the incident 

 light, has several times been determined. 



It varies somewhat with the kind of plate, but «ith plates 

 of medium speed developed with a non-staining developer it 

 may be taken as "0001. So that 1 gram of silver can produce 

 a density of U over a square metre of photographic plate. 



The photometric constant has been pro\ed to be rigidl\- 

 independent of the Density. 



For a given exposure the relation bclween the Density 

 produced and the time of development is given by an 

 exponential fornuila — 



D = C (1 - e-'") 



where k is the velocity constant of development, and C is the 



and 



is termed the Transparency of the plate (T) 



the Opacity (OL 



