626 



NA TURE 



[October 27, 1892 



great — or, in other words, the microscope tube being exceed- 

 ingly long as compared with the table instrument — the objective 

 has to be approached very close to the slide ; in fact, with the 

 higher powers, closer than the cover-glass will allow. This close 

 working distance renders necessary special sub-stage condensers, 

 and in many cases a special one is required for every screen 

 distance with each objective. This requisite would seem to be 

 a complete stumbling-block to microscope projection work. 

 With the lime-light the difficulty does not enter, in the same 

 degree as with the arc light, and as we are now dealing with 

 the latter, further reference need not be made to the oxy- 

 hydrogen light. There are two ways of surmounting 

 the difficulty ; one by the use of plano-concave lenses, 

 introduced in such a way as to be equivalent to greatly 

 lengthening the focus of the objective on the screen side, 

 while it enables, as a consequence, the objective to be slightly 

 further removed from the slide, i.e., giving what is termed a 

 greater working distance. The objection to this method is that, 

 even when these plano-concave lenses tare corrected, the result, 

 though greatly improved, is not perfect. The second way, 

 which is a perfect one, is that of introducing an eye-piece. In 

 both these methods, that the best results may be obtained, the 

 objective is made to occupy a position not very different from 

 that which it would do if employed on the table microscope. 



In the eye-piece method almost the exact conditions can be 

 complied with for which the objective was made. I propose, 

 therefore, to show the subjects by the eye-piece method. The 

 only objectives which will be used are : (l) Zeiss's 35 millimetre 

 projection objective, with a sub-stage condenser, 4 inches focal 

 length, placed a considerable distance from the slide ; (2) New- 

 ton's i-inch projection objective, the sub-stage condenser as in 

 the first case ; and (3) Zeiss's J-inch achromatic objective, the 

 sub. stage condenser being Prof. Abbe's three-lens condenser 

 with the^lront lens removed. In all three cases the eye-pieces 

 used are Zeiss Huyghens No. 2 and No. 3. 



In each instance I will mention the magnification in diameters, 

 as well as the number of times when reckoned by area, for the 

 appreciation of those who estimate by area ; and I will also give 

 the size to which a penny postage stamp would be increased, 

 supposing it to be made of India, rubber, and stretchable to any 

 extent in all directions. In presenting these figures I do not 

 pretend that they are absolutely correct, but as they have been 

 ascertained under conditions similar to those now existing the 

 errors will not be very great. 



In consequence of the field not being quite flat, and the sec- 

 tions having a certain thickness, although extremely thin in 

 most cases, the whole of the object cannot be in rfocue upon the 

 screen at the same time. By shifting the focussing screw slightly 

 all parts may be brought into focus successively. So-called 

 greater depth of focus is obtained by using an increased working 

 distance ; and for projection work over- correction for flatness 

 can alone give a sharp picture all over with very considerable 

 depth of focus ; the difficulty of over-correction being that, un- 

 less extreme care is taken, certain forms of distortion may be 

 introduced. By stopping down the objective greater flatness of 

 lield may be secured, but at the expense of light. There is thus 

 a choice of difficulties, and the least one should be taken. 



Turning now to the polariscope. Polarized light teaches us 

 a great deal concerning the structure of matter ; it is also a 

 means of confirming the undulatory theory of light. This sub- 

 ject is «o large that no attempt can be made to give even a 

 general idea of the field it covers, and the experiments, which 

 will be shown in the polariscope, may be taken simply as a few 

 illustrations of the subject and nothing more ; but they will, at 

 any rate, be suggestive of the large field to which this method 

 of analysis can be applied. A vast amount of mathematical 

 proof can be illustrated graphically by various experiments with 

 polarized light. I will show on the screen a diagram of the 

 polariscope. (Shown.) 



With reference to showing the spectrum. The method of pro- 

 jecting a spectrum, I think, is new, as I have not seen it de- 

 scribed anywhere. It gives practically a direct spectrum with an 

 ordinary prism, without turning the lantern round to an angle 

 with the screen ; and here is a dia'^ram of the method. 



The details of the apparatus, as well as those of the methods 

 of working, I have modified in almost every instance, for five 

 reasons: — (i) That more certain results, may be ensured ; (2) 

 that rapidity may be obtained ; (3) that only one operator may 

 be needed ; (4) that, as far as possible, all parts of the apparatus 

 may be interchangeable ; and (5) that loose screws and pieces 

 may be dispensed with. 



NO. 1200, VOL. 46] 



There were then shown by projection a number of slides illus- 

 trating various microscopic optical systems, and a number of 

 microscopic slides, followed by a series of general polariscopic 

 projections, some of them to illustrate the strains existing in 

 many forms of matter ; also a spectrum by a carbon disulphide 

 prism, in conjunction witn a reflecting prism and with a mirror, 

 which, apart from any other result, demonstrates that the loss of 

 light with a reflecting prism is less than with an ordinary glass 

 mirror. Slides and other projections were also thrown upon 

 the screen. 



The details are as follows : — 



The Microscope. — Screen distance, 21 feet. First 35 milli- 

 metres Zeiss projection objective, 4-inch sub-stage condenser, 

 Zeiss Huyghens eye-piece 2 ; 500 diameters = 250,000 times = 

 penny stamp stretched to cover about 147 square yards. Subjects 

 shown : proboscis of blowfly ; permanent molar displacing milk- 

 tooth (kitten) ; human scalp, vertical ; human scalp, surface ; 

 fossil ammonites and belemnite. Second, i- inch Newton's pro- 

 jection objective, 4-ineh sub-stage condenser, Zeiss Huyghen's 

 eye-piece 2 ; 1,000 diameters= 1,006,000 times = stamp stretched 

 to about 588 square yards. Objects shown : proboscis of blow- 

 fly ; foot of a caterpillar ; section of human skin, showing the 

 sweat ducts • phylloxera vastatrix of the vine. Third, i-inch 

 Newton's projection objective, 4-inch sub-stage condenser, 

 Zeiss Huyghens eye-piece 3 ; 1,300 diameters = 1,690,000 

 times = stamp stretched to about one- fifth of an acre. Slides 

 shown : proboscis of hlow-fiy ; wings of bee (showing booklets 

 and ridge) ; sting of bee (showing the two stings, sheath, and 

 poison-sack) ; sting of wasp (showing same as last slide) ; eye of 

 beetle (showing the facets). Fourth, J-inch Zeiss's achromatic 

 objective ; Abbe's 3-lens sub-stage condenser, with top lens re- 

 moved ; Zeis5 Huyghens eye- piece 3 ; 4,500 diameters = 

 20,250,000 times = stamp extended to nearly 2^ acres. Slides 

 shown : proboscis of blow-fly ; hair of reindeer (showing cell 

 structure) ; hair of Indian bat (showing the peculiar funnel-like 

 structure) ; sting of bee (showing the barbs) ; foot of spider ; 

 stage of the micrometer (the closest lines ruled to thousandth of 

 an inch, which measure 4^ inches apart under this magnifica- 

 tion) ; a wave length -^^J^^inch, therefore, on screen measures 

 about ^-inch. 



The Polariscope. — Shown with parallel light: plain glass; 

 glass under pressure ; chilled glass (round, oval, and waved peri- 

 pheries) ; Prince Rupert's drop (broken in the field) ; horn ; 

 selenites (over-lapped) ; butterfly (selenite) ; bunch of grapes 

 (selenite) ; bi-quartz, with J-wave plate (the ^-wave plate in 

 this experiment produces the same effect upon the bi-quartz as 

 if a column, 20 centimetres long, of a ']\ per cent, solution of 

 cane sugar were placed between the polarizing nicol and the bi- 

 quartz. The analyser has to be rotated about 10°) ; a piece of 

 sapphire to show asterism. Shown with convergent light ; 

 hemitrope (cut in a plane, not at right angles to the axis) ; ruby 

 topaz ; grape sugar (diabetic) ; cane sugar ; quartz ; superposed 

 right and left-handed quartz (spirals) ; calcite and phenakite 

 superposed (■-bowing transition from negative to positive crystal, 

 passing through the apopholite stage). 



The Solidiscope. — New form of apparatus for showing solids, 

 and consisting of two reflecting prisms and suitable projecting 

 lenses. With this instrument were shown : — Barton's button, 

 the works of a watch, a coin. 



Spectrufn Analysis. — Spectrum thrown by means of a disul- 

 phide prism combined with a reflecting prism ; the result being 

 that a good spectrum is thrown upon the screen direct without 

 turning the lantern. There were shown : — The spectrum ; 

 absorption bands of chlorophyll, &c. ; effects produced by pass- 

 ing the light through coloured gelatine films. 



Projection of Slides. — Decomposition of water ; expansion of 

 a wire by means of heat ; combination of colours to form white 

 light; various diagrams, coloured photographs of a workshop, 

 &c. As an extra experiment there was shown, in the polaris- 

 cope, with a convergent light, Mitscherlisch's experiment (illus- 

 trating the changes which take place in a selenite under the 

 influence of heat). 



There are but few who would disagree with me in the opinion 

 that the microscopic world, as regards its design and its mole 

 cular structure, is quite as wonderful as the great works aroun* 

 us seen with the unaided eye. A magnifying glass of low powei 

 opens up a world far larger than that which we are accustomed 

 to see. At the present time, even with the most perfect appa 

 ratus that exist, only a small portion of the universe is known t ■ 



