November, 1912. 



KNOWLEDGE, 



439 



power than si.xtecn milliinetres must be specially corrected for 

 uncovered objects. When oil immersion lenses are employed 

 the specimens may then be covered. In order to reduce 

 reflections upon the lens surfaces as much as possible, the 

 objectives should be mounted in a manner which allows the 

 back lens to be as close as possible to the prism. With this 

 form of illuminator there are usually supplied thin metal 

 diaphragms which slide into slits cut in the body tube ; their 

 use is to cut off reflections which give rise to a hazy 

 appearance in the image, and, being adjustable from right 

 to left, the position in which the image is seen at its 

 best must be found by cxpciiiiient. It i> als.) advisabU- 

 to reduce the im- 

 age of the source 

 of light " by means 

 of an iris dia- 

 phragm," so that 

 it is not larger 

 than the field of 

 view of the ob- 

 jective. As a deal 

 of care is necessary 

 in the adjustment 

 of the light in 

 order to get the 

 best effect, special 

 microscopes have 

 been designed for 

 use in metallo- 

 graphy. These 

 instruments have 

 an adjustment for 

 raising and lower- 

 ing the stage, and 

 as a fine adjust- 

 ment is provided 

 as well, the whole 

 of the focusing is 

 performed by this 

 means, thus avoid- 

 ing any shifting of 

 the light by move- 

 ment of the tube 

 of the microscope. 

 Excellent results 

 can. however, be 

 got with an or- 

 dinary microscope, 

 although more time 

 is occupied in ob- 

 taining them, and 

 in the illustration 

 (Figure 470) we 

 have an example 

 showing a photo- 



micregraph of the etched surface of some cast steel, in 

 which an ordinary microscope was employed, and light from 

 a paraffin oil lamp the source of illumination. 



PHYSICS. 



By .Alfred C. G. Egerton, B.Sc. 



RESONANCE SPECTRA.— Professor Wood has con- 

 tinued his researches during the last summer on the resonance 

 spectra of iodine vapour when it is excited by waves of light 

 of different wave-length. His former work had showed that 

 a very small difference in the wave-length of the exciting 

 light caused a very great difference in the iodine spectrum ; 

 the latter differs when it. is excited by the green line of the 

 Cooper Hewitt mercury lamp made of glass from the spectrum 

 obtained when excited by the same line from a similar lamp 

 made of (juartz but working at a higher temperature. It 

 appeared that the exciting line might be broad enough to act 



Figure 470. 



Cast steel X 220 diameters. Photographed with an eight millimetre objective and 

 2eiss vertical illuminator, together with a four projection ocular. 



upon more than one absorption line. The spectrum of the 

 light absorbed by iodine vapour is very complicated. Professor 

 Wood has estimated the number of absorption lines at fifty 

 thousand ; seven distinct lines were visible within the green 

 mercury line, the wave-lengths varying from 5460-966 to 

 5460-579. To be able to make these measurements of wave- 

 length with such remarkable accuracy. Professor Wood has 

 constructed a very powerful spectrograph and as befits his 

 well-known ingenuity, he has made it in a soniewh.it novel 

 manner. The large plane grating was mounted on a cast iron 

 pilbr outside the laboratory, the slit and photographic plate- 

 liiildrr was lixt-ci oil aiKithrr niiT inst inside; these piers con- 

 sisted of jointed 

 water mains, while 

 ;i bevel gear taken 

 from an old hand- 

 drill served to ro- 

 tate the grating. 

 The lens was one 

 of forty feet focus 

 and threw the 

 spectrum on to 

 the plate. Rect- 

 aTi^'ular tubes made 

 (if wood connected 

 the different parts 

 of the apparatus, 

 but they were not 

 rigidly joined up 

 so as to prevent 

 errors arising from 

 vibrations set up 

 by wind or other 

 source. This spec- 

 trograph, though 

 constructed in 

 such a simple man- 

 ner, is probably 

 the most powerful 

 in the world. One 

 interesting result 

 obtained with this 

 instrument is that 

 there appear to 

 be many coincident 

 lines in the ab- 

 sorption spectrum 

 of iodine and of 

 bromine its kin 

 amongst the ele- 

 ments. This would 

 suggest that pos- 

 sibly there exist in 

 the two elements 

 identical systems 

 hich give rise to similar frequences in the 



of electrons, v 

 two molecules. 



ABSORPTION SPECTRA.— Much work has been done 

 on the absorption spectra of organic compounds, similarity in 

 the position of the maximum of absorption being taken as a 

 clue to similarity of structure of the molecule. The origin of 

 the colour of an organic substance is supposed to be due to 

 certain groups within the molecule called chromophoric 

 groups ; the simpler substances containing the chromophoric 

 group usually have absorption bands in the ultra-violet or 

 violet, so that they are yellow in colour and as the molecule 

 gets heavier, with the addition of other groups the positions of 

 the absorption bands shift towards the red end of the 

 spectrum, consequently the colour of the compound becomes 

 redder and then purple and finally blue. The mutual attrac- 

 tions between the groups within the molecule cause alterations 

 in the vibration of the chromophoric group which is respons- 

 ible for the absorption of the light, and the colours do not 

 usually quite go through the series of colours that should 

 theoretically be yielded by the formation of substances 



