138 



KNOWLEDGE lV SCIENTIFIC NEWS. 



[June, 1905. 



Cunditcted by F. Shillkngton Scales, f.k.m.s. 



Photo-Micrography with Ultra-Violet 

 Light. 



The resolution of a microscope objective is deter- 

 mined by its aperture, and though the use of immersion 

 objectives has enabled us to increase the latter, and, 

 consequently, the former also, to an extent undreamt 

 of in the days when we were limited to the use of 

 lenses used drj", a suitable all round medium has net 

 yet been discovered which will satisfactorily replace 

 cedar oil and so enable objectives to be made of still 

 greater aperture. True, an immersion lens has been 

 made which is used with mono-bromide of naphthalene, 

 and which has a proportionately greater aperture even 

 than those used with cedar oil, and, therefore, greater 

 powers of resolution, but this medium is, unfortunately, 

 not a suitable mounting medium for most objects, so 

 that the lens is but little used. Without entering into 

 the diffraction theory, it may be stated that with an 

 objective of given aperture — say 1.4 X..'\., w'hich is 

 approximately our present practical limit — we can only 

 increase the resolution by reducing the velocity of the 

 light by which we illuminate the object — that is, by 

 increasing the refractive index of the medium in which 

 the object is mounted — for example, when it is mounted 

 in realgar, or by using light of shorter wave-length. 

 The first method has, of course, its limits as already 

 mentioned, but there remains the second. Now, it is 

 well known that white light is made up of rays of 

 different refrangibility, and, accordingly, of different 

 wave-lengths, of which those at the red end of the 

 spectrum are the longest, and those at the violet end 

 the shortest. Therefore, mono-chromatic light, 

 selected by means of a prism or screen, and taken from 

 the blue, or, still better, the violet end of the spectrum, 

 will give us greater resolution than ordinary white 

 light which combines so many rays of longer wave- 

 lengths. This fact is taken advantage of in photo- 

 graphy, and the result is a very definite increase in 

 resolution, say, of the markings of a difficult diatom. 

 U'hen violet light is used the eye is, unfortunate!}', 

 little sensitive to these rays, so that it is not easy to 

 see the object, and though such light has high actinic 

 value, it is difficult to focus the object satisfactorily 

 when it is used. Dr. Kohler, of Jena, has, therefore, 

 experimented with ultra-violet rays, which are in- 

 visible, but can be used for photography, and their 

 still shorter wave-length, 275 ^m. has proportionately 

 greater resolving powers. The lenses are made of 

 crystal and fused quartz, and as mono-chromatic light 

 is to be used, they need correction only for spherical 

 and not for chromatic aberration. The light is ob- 

 tained from electricity passing between cadmium 

 electrodes. But the human eye, as we know and have 

 just stated, cannot sec these rays, and so cannot focus 

 and adjust them. Therefore, Dr. Kohler has devised 

 what may be called an artificial eye; in other words, he 

 constructs what corresponds to an eye-lens, made of 

 crystal, and a retina made of fluorescent glass, which 



responds to these ultra-violet rays. The image on this 

 ■' retina " is examined visually by means of a lens, in 

 which case Dr. Kohler has found magnesium light, of 

 wave-length 280 M/i, better than the cadmium light. 

 The fluorescent light, however, is, unfortunately, harm- 

 ful to the eye, and, apart from this, the best results 

 are given by photography. The objects, mostly 

 organic tissues, have been so far mounted in dilute 

 glycerine or in salt solution, and structure has been 

 made evident, which, before, required staining to 

 bring out, morv? especially because of the comparative 

 impermeability of certain structures, such as the horny 

 layer of the skin, and plant membranes. The lens and 

 its adjuncts were made by the firm of Zeiss, and has 

 recently been exhibited at the Natural Science Club in 

 Cambridge. It may have considerable possibilities. 



Royal Microscopica.1 Society. 



April 19, at 20, Hanover .Square, Dr. Dukinfield H. 

 Scott, F.R.S., President, in the chair. The Secretary 

 read a description c;f an old portable microscope made 

 by W. and S. Jones, which was said to have been the 

 pocket microscope of Dr. Jenner. Mr. W. J. Dibdin 

 exhibited a slide of Bacillus typhosus., and explained 

 the method adopted in staining and mounting. He 

 also exhibited photo-micrographs of the slide at magni- 

 fications of 2,500 and 5,000 diameters, with the flagella 

 well displayed. Mr. .'\. E. Conrady gave a resume of 

 his paper, " On the Application of the Undulatory 

 Theory to Optical Problems," illustrated by diagrams 

 shown upon the screen. Dr. Spitta said that in using 

 the method of graphical representation, Mr. Conrady 

 rendered the subject intelligible to most people, and 

 inquired if the method of explaining the subject 

 originated with the author, as he did not remember 

 having seen it in any of the text-books. Mr. Conrady 

 said the method was not devised by himself, but would 

 be found in the article on the Wave Theory, by Lord 

 Rayleigh, in the " Encyclopaedia Britannica." 



Quekett Club Journa.!. 



1 he h:ill-vearlv issue ol lliis j()urii:il contains ralher 

 less matter than usual. It appears from the .'\nnual 

 Report that owing to an increase of rental it was found 

 necessary to economise in the Journal by shortening 

 the reports of meetings — which is probably not much 

 loss — and in omitting the reviews of books, which were 

 a useful feature. However, I am glad to see that the 

 membership has considerably increased during the past 

 year, and that the finances of the Club are in a satis- 

 factory condition. The Journal contains Dr. Spitta's 

 address on " Improvements in Modern Objectives," a 

 translation by Mr. Rheinberg of Prof. Ambronn's re- 

 view of Prof. Abbe's work, and other papers and notes. 



Watson-Conrady Photo-Micrographic 

 Apparatus. 



Mr. A. E. Conrady has computed for Messrs. W. 

 Watson and Sons an entirely new system of lenses for 

 photo-micrography, which I have found to give better 

 results than any other apparatus which has come under 

 my notice. It is mounted in the modern way on an 

 optical bench, by means of which each part is capable of 

 ready adjustment whilst keeping in exact alignment, 

 centring screws being provided for the preliminary ad- 

 justments. Taking the parts in order we have first a 

 source of illumination. This may be by arc light or by 

 oxy-hydrogen jet, mechanical adjustments, both vertically 

 and horizontally, being provided, and the whole enclosed 



