October, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



573 



Parallel Light in Photo-micrography. 



An inquiry from a correspondent in Melbourne, 

 Australia, raises a point in illumination for photo- 

 micrography which puzzles many people and is worth 

 my answering in detail. If a well-corrected stand con- 

 denser is adjusted so as to give parallel light, a well- 

 corrected sub-stage condenser will then give an image 

 of the source of illumination upon the screen, if the 

 sub-stage condenser is fcx-ussed on the object on the 

 slide and the objective correctly focusscd likewise. 

 Using the Nernst incandescent filament a narrow streak 

 corresponding tO' the very narrow filament will be vis- 

 able, w hilst the edge of a l.-unp-wick would give a corre- 

 spondingly broad streak. Any statements that are 

 met with in text-books as to obtaining a fully-illuminated 

 field with parallel light by this means are doubtless 

 based on practical experience and are due to the fact 

 that ordinary stand condensers are so ill-corrected that 

 a perfect representation of the image is not obtainable. 

 For visual work nothing better than this streak could 

 be wished for; the streak image, ugly as it appears 

 to the uninitiated, being the only really critical form 

 of illurnin.ilinn for all critical work, for, .after 

 .-ill, it is only .'i small part of the field that 

 the worker is actually investigating, and for 

 this he must ha\e the utmost possible degree of 

 resolution and definition. In photo-micrographv, how- 

 ever, such a streak would be unpleasant, and, there- 

 fore, resolution must be sacrificed sliglitlv for the sake 

 of even illumination. To do this either the sub-stage 

 condenser must be thrown very slightly out of focus, or 

 the stand condenser must be re-adjusted. My own 

 method is to use the stand condenser to focus the .source 

 of light in the air about eight inches from the sub-stage 

 condenser, and in this position I place an iris 

 diaphragm. The sub-stage condenser is then focussed 

 on this iris diaphragm, and consequently on the image 

 of the source of illumination. A verv sliglit adjustment 

 to the sub-stage condenser will then give me a bar of 

 critical light, or enable me to fill the field uniformly, 

 just as if I were focussing directly on the lamp. But 

 it is only the very best sub-stage and stand condensers 

 that will do this, and of the latter I know of no other 

 condenser than Mr. Conrady's which is suflicientlv well 

 corrected. 



Having adjusted the stand condenser for parallel 

 light, on looking directly at it an image of the source of 

 illumination will be seen because the eve itself is able 

 to bring to a focus parallel rays. But if the condenser 

 be unable to form trulv parallel rays, it will be seen to 

 he filled with light. That such parallel r.-ivs are not 

 formed by ordinary condensers is readily demonstrated 

 by endeavouring to adjust the stand condenser so that 

 on bringing a sheet of paper nearer and removing it 

 further the disc of light upon it remains of the same 

 diameter, as it should do if the light were really parallel. 

 The articles on photo-microgr.aphy which I contributed 

 to these columns from November, T905, to June, 7906, 

 inclusive, will probably be foimd of ser\ ice on this and 

 .similar points. 



New Micro-chemical Tests for Wood. 



_The Journal of the Royal Microscopical Society 

 gives the following summary of some new micr(>- 

 chemical tests for wotxl, by V. Graefe. He adds to a 

 solution of vanillin some drops of isobutyl-alcohol, and 

 lets a little sulphuric acid (sp. gr. i. 84) run down the 

 side of the test-tube. .After heating, the mixture turns 

 dark red, with a shade of blue. " On diluting with 

 alcohol and repeated additions of acid, it changes 



through blue-green to pale green. The author recom- 

 mends as a standard re-agent the following : — 30 c.cm. 

 isobutyl-alcohol p/us 15 c.cm. sulphuric acid. When 

 wood-mash is treated with this re-agent the wood turns 

 black; if now diluted with a little alcohol and the test- 

 tube be shaken, the wood turns blue or blue-green, 

 whilst the fluid becomes red-violet. .Sections of 

 ligneous tissue treated with this fluid are at first red- 

 violet and after a time blue. The sections .should re- 

 main in the re-agent for about an hour and then be 

 mounted in glycerine. .Apparentlv the stain is not verv 

 permanent. A mixture of isobutyl-alcohol and sul- 

 phuric acid also forms a useful re-agent. A drop of 

 the mixture placed on a micro-section gradually turns 

 It red, and if, after the lapse of about an hour, it be 

 placed in glycerine it assumes a wine-red or red-violet 

 hue. 



Quekett Microscopical Club. 



.'\ series of demonstrations on the practical use of the 

 microscope and its accessories has been arranged for 

 the ensuing session. The first will be given by H. F. 

 Angus on November id, on " .Axial Sub-stage Illumina- 

 tion," embracing the use of the plane and concave 

 mirrors, sub-stage condensers, methods of centring the 

 illuminant, and critical illumination. Subsequent 

 demonstrations will deal with sub-stage non-axial 

 illumination, including oblique, dark-ground, and 

 multi-colour illumination, the employment of polarising 

 apparatus, illumination of opaque objects, the use of 

 various pieces of apparatus for recording observations, 

 &'C. .Arrangements are not yet completed, but a full 

 programme will be available shortly. The demonstra- 

 tions will be given on the third Friday in each month 

 from 7 till S p.m., and will not interfere with, but will 

 be in addition to, the ordinary meetings of the Club, 

 which are held at 20, Hanover Square, U'., on the third 

 Friday in each month at 8 p.m. precisely. Gentlemen 

 interested are invited to communicate with the Hon. 

 Sec, A. I'larland, ;,t, f^Jenmark Street, Watford, Hi-rts. 



Microscopical Lectures. 



The Manchester Microscopical Scx-ietv has sent me 

 the prospectus of its Extension Lectures for the forth- 

 coming winter, and I am glad to again call attention to 

 so excellent a scheme. The prospectus contains a list 

 of fifty-three lectures by various members of the Society 

 on almost all subjects coming within the range of the 

 microscope, many with titles no less attractive than 

 the subjects of the lectures, and all illustrated by 

 lantern slides, specimens, microscopes, or drawings. 

 The object of the extension scheme, now in its ninth 

 year, is, as the prospectus states, "to bring .scientific 

 knowledge, in a popular form, before societies who are 

 unable to pay large fees to professional lecturers," and 

 this is done w ithout other charge than that of travelling 

 expenses and hire of slides, save where lectures are 

 given before societies which are commercial under- 

 takings, or are subsidised from public grants. These 

 lectures must be of great value in the populous working- 

 class districts of Manchester and its neighbourhood, 

 and reflect both credit and honour on the energetic 

 society which is responsible for them, and, as I have 

 said before, it would be well if other S(x:ieties would 

 follow so excellent an example. They would them- 

 selves benefit much by coming in contact with a wider 

 public than that of their own membership. 



[Communkalions ami Enquiries on Microscopical niatlcis should he 

 addressed to F. Sliillingtou Scales, "Jersey" St. Barnabas Road. 



Cambridge.] 



