Februaky, 1903.] 



KNOWLEDGE 



45 



the specimens inside to dry, a matter of no small moment if 

 the risk of mildew is to be avoided. After standing thus for a 

 few days the envelopes can be safely folded over at the ends 

 and put away until a convenient opportunity occurs for 

 examining the contents, when soakins; in hot water will speedily 

 restore the plants to their original freshness, though not, of 

 course, to life. The ease with which mosses can in this way be 

 revived for examination constitutes, to my mind, one of the 

 chief attractions which this branch of botanical research offers 

 to anyone in search of a hobby, for while the gathering of 

 specimens forms a healthy out-door occupation for all seasons 

 of the year, and adds immensely to the pleasure and interest of 

 a ramble in the country, their examination and mounting may 

 be deferred for any length of time, and will provide the most 

 pleasurable recreation by the fireside in the long winter evenings, 

 and will bring back the memory of many a happy day in the 

 woods and fields. 



The nature of the apparatus used in the examination of 

 specimens will, to a large extent, depend upon the views and 

 means of the student. A microscope of some kind is essential 

 if the study is to be followed up with any degree of thorough- 

 ness, though much good work may be done with a comparatively 

 inexpensive instrument ; at the same time it is no doubt true 

 that, within certain limits, the better the implement the better 

 the results to be obtained by its use. My own experience would 

 lead me to say that, if the funds will allow, the binocular model 

 is greatly to be preferred to the monocular, if only on account 

 of the saving of strain to the eyes that is thereby effected. 

 Objectives of high power are seldom required ; I generally work 

 with 2 in., 1 in., and i in. lenses, to which may be added a .3 in., 

 which is especially useful in the exhibition of slides. To prepare 

 specimens for examination or for mounting, some form of 

 dissecting microscope is practically a necessity. For many years 

 I used an ordinary magnifying-glass, of low power, fixed in a 

 light metal frame, at one end of which was a small collar, which 

 slipped over a screw fixed in an upright position in a small metal 

 stand, and provided with a nut by means of which the lens 

 could be fixed in a horizontal position at any required height, 

 and this simple expedient is still often very serviceable. As a 

 rule, however, I now employ the more modern binocular form 

 of dissecting microscope, which is also of the greatest assistance 

 in mounting. For soaking the mosses in hot water I have found 

 nothing so handy as the small china saucers, made in different 

 sizes, and sold by artists' colourmen. For dissecting purposes 

 ordinar}' sewing needles set in cedar pen-holders are useful in 

 the more delicate work, and it is well to have one or two bent 

 at an angle to the holder, for the purpose of altering the position 

 of objects under examination after the cover-glass has been put 

 on ; these, moreover, will be found invaluable in subsequent 

 mounting operations. In order to bend a needle into almost 

 any form it is only necessary to heat it in a spirit lamp to a red 

 heat, and then plunge it into water, this will render it soft and 

 pliable. The best dissecting implements, however, for ordinary 

 work that I know of are made by fixing glovers' needles into 

 handles in the above manner. I use Mogg & Co.'s glovers' 

 needles. No. 4. They can be obtained at a small cost at any 

 drapers, and are not only ])rovided with a fairly good point, but 

 are ground with three flat faces, thus giving as many cutting 

 edges, by the help of which the specimen can be most effectually 

 prepared for examination and mounting. A pair or two of 

 forceps (one with curved ends), a pair of small scissors, a small 

 camel's-hiiir brush, and one or two small lancets will practically 

 complete the implements required for all ordinary dissecting, 

 to which must be added, for the pur])ose of microscopical 

 examination and subsequent mounting, a stock of the usual 

 glass slips (.3 in. by 1 m.), and a few sizes of cover-glasses. 

 With many of the larger mosses the square cover-glass (J in. 

 and :l in.) is best, as it gives more mounting surface, while for 

 the mounting of pieces of moss of considerable size, cover- 

 glasses that are specially large should be procured. 

 ( To he cnnlhiued. ) 



PHOTODRAPHY OF OPAQUE OBJECTS. 

 By Fkedekick No.vd Ci,.\rk. 



(Cimlinueil from pa/je 20. j 

 Perhaps the most useful all-round objective for this work is 

 the 2-inch ; a lower power than this we have found of no 



advantage when depth of focus is an object. The use of a stop 

 in the form of an iris diaphragm or a Davis' shutter is some- 

 times necessary, it being fitted just above the objective. This 

 will increase the depth of focus when photographing spherical 

 objects or those lying in different planes ; but the expo.sure is 

 necessarily prolonged. 



A plate giving the maximum amount of density is necessary. 

 For this purpose any of the well-known makes of isochromatics 

 are best. They must invariably be "backed." The developer 

 should be dilute, and if pyro-soda is used (which we recommend) 

 the pyro should be used sparingly. Development ought to be 

 slowly carried on to full density, the aim being to obtain a 

 negative with plenty of contrast, without blocking of the high 

 lights. It is to give this effect that the backed plate is so 

 essential. 



As to length of exposure no exact information can be given. 

 As a rough guide it has been found that using a 2-inch objective 

 with direct illumination by means of .the bull's eye from a good 

 paraffin lamp with a 1-inch wick, the exposure at 20 diameters 

 will vary from 2 to ?> minutes according to the colour of the 

 object ; a white object naturally requiring a less exposure than 

 a darkly-coloured one. As further examples we may mention 

 those of Forarainifera at 8 diameters with a 2-inch requiring 

 30 seconds exposure, whilst Polycistina at -t') diameters and a 

 1-inch objective required .'^0 minutes. 



When photographing objects too large for the field of a 

 micro-objective, lenses of the Planar type will be found con- 

 venient. These, in conjunction with a long-extension camera, 

 will give a magnification of from 2 to 10 diameters. 



The eggs of insects, particularly those of the Lepidoptera, 

 lend themselves admirably to photography. Especially suitable 

 are those of the Lycaenida;, or blue butterflies, their small size 

 and elaborate structure rendering them most beautiful objects 

 for opaque photography. 



We would impress upon the worker tbe advisability of photo- 

 graphing these objects at the same nngnification whenever 

 possible, in order to give a proper idea of their relative sizes. 

 Two or more aspects of the same object are sometimes desirable, 

 as in the case of ova, where an upright and a lateral view are 

 frequently of importance. 



Many other subjects for opaque photography might be men- 

 tioned, such as the hairs and seeds of botanical specimens, fern 

 spores and micro-fungi, whilst some of the larger Diatoms, 

 Foraminifera, and Polycistina are eminently suitable for this 

 class of work. 



WoRKlxu WITH A PoLARiscoPE. — The number of micro- 

 soopists who use the polariscope for scientific work is exceed- 

 ingly limited, by comparison with those who find in this 

 adjunct a never-failing source of pleasure and variety, but 

 whether it be used for colour effects or for scientific observa- 

 tions, there can be but one opinion as to the disadvantages pro- 

 duced by the prisms, and particularly when the analyser is used 

 over the eyepiece. It seems well-nigh impossible to get a prism 

 of moderate size which will fill the whole field of the eyepiece, 

 and there is invariably a disagreeable dark patch on the margin 

 of the field. Further, the necessity of working with the eye 

 some distance from the eyepiece lens reduces, in some measure, 

 the sharpness of effect. 



In a lesser degree, the polarising prism interferes with the 

 best performance of the substage condenser, and working is 

 ))ractically limited to condensers having small lenses, otherwise 

 the prism would act as a diajihragm. 



Mr. E. M. Nelson communicated a sjiecial note on this sub- 

 ject to the April (l',)02'l .hntrmil of the It'oyal Mirro.tcopifal 

 Soc'etj/, and therein indicated the advantages of using tour- 

 malines instead of prisms. 



One tourmaline (which need not be of best quality), measuring 

 about 4/ 10th by ('>/ 10th, should be mounted in a metal screen, 

 with an aperture of such a size as to prevent any light passing 

 from the lam|i, except through the tourmaline. This screen 

 may be held on a separate stand, or in an extra frame attached 

 to the lamp chimney. This would act as the polariser, and the 

 image of the lamp flame could be focus.sed with the condenser, 

 in the usual manner. Auother tourmaline, wliich must be as 

 perfect as can be obtained of "smoky tint, with the slightest 

 dash of pink." is fitted over the eyepiece. With this in use, the 

 best ett'ects obtainable with the" polariscope will be obtiiined, 



