June 8, 1905] 



NA TURE 



^n 



Provision has been made in the Navy Estimates for the 

 observation of the total solar eclipse of 1905 August 30 

 by a party of three observers on the coast of Tunis, where 

 the weather conditions are promising. It is proposed to 

 take photographs of the corona for detail and extension 

 with the Thompson g-inch coronagraph, the 13-inch astro- 

 graphic refractor, and the 4-inch Dallmeyer rapid recti- 

 linear lens, and also photographs of the spectrum with 

 the two spectroscopes lent by Major Hills, R.E., as in 

 1900 and 1901. 



THE OPTICAL CONVENTION. 

 'T'HE optical convention has just concluded a very 

 ■*■ successful meeting extending over four days ; the 

 exhibition and the papers attracted numerous visitors from 

 all parts of the country. The papers led to much valuable 

 discussion. An account of the exhibition and the presi- 

 dent's address appeared in last week's Nature. 



The first group of papers dealt with the design of 

 optical and scientific instruments. The Gauss theory 

 was entrusted to Mr. Conrad Beck, who considered the 

 theory of the equivalent planes of complete optical instru- 

 ments ; he dealt more particularly with the complete 

 microscope in relation to its " working distance." 



Dr. Drysdale gave a general account of the aberrations 

 of lens systems, submitting a classification and specifi- 

 cation of the various aberrations to obtain an expression 

 of opinion from those working at the subject. Mr. 

 Chalmers gave a graphical method of representing the 

 results of calculations of lens systems, and a modification 

 of the Hartmann system of testing to permit of measuring 

 and expressing aberrations in exactly the same form. 

 This should make it possible to obtain the relation between 

 the definition and the measured or calculated aberrations. 

 In the discussion Mr. Carson pointed out the importance 

 of the relative intensity of the image disc and the aberration 

 patch in estimating the performance of lenses. 



Mr. Walter Rosenhain criticised the mechanical design 

 of certain types of instruments ; he showed that, in many 

 cases, the ideals of the instrument maker were in conflict 

 with sound engineering principles, and suggested directions 

 in which improvements might be looked for. 



Diffraction in optical instruments was discussed by Mr. 

 J. W. Gordon in an important theoretical paper; his con- 

 clusions, which would modify many of our ideas on optical 

 systems, are now being submitted to a definite experimental 

 test. 



A group of papers related to interference phenomena. 

 Mr. J. Rheinberg exhibited a method of producing achro- 

 matic interference bands which is likely to have numerous 

 applications. Mr. Stansfield described a simple form of 

 Michelson interferometer specially suitable for demonstra- 

 tion. Prof. Watkin and Mr. Morrow exhibited their 

 apparatus for calibrating extensometers by observing the 

 displacement of interference bands. 



Mr. Twyman described the manufacture of the Echelon 

 spectroscope, stating the accuracy .required in the plates 

 and the precautions used to obtain it. This apparatus 

 was exhibited and compared with the Lummer " parallel 

 plate " arrangement for obtaining resolution of spectrum 

 lines. Mr. Blakesley discussed the various forms of prism 

 which could be used in constant deviation spectroscopes 

 and some of their applications. Mr. Newall dealt with 

 astronomical spectroscopes, demonstrating that the limits 

 of usefulness of the present type of spectroscope were 

 almost reached in the case of faint stars, as the intensity 

 of light necessary for photographing their spectra can only 

 be obtained at the sacrifice of the purity of the spectrum 

 or the certainty of identification of the lines, and that no 

 very marked improvement is likely to be obtained from 

 the use of larger objectives, on account of the increased 

 absorption in the prisms required. He suggested the use 

 of gratings. 



Lord Rayleigh dealt with the subject of polish, pointing 

 out the distinction between the process of grinding, which 

 consists of the removal of comparatively large flakes, and 

 that of polishing, which he regards as molecular, the rough- 

 nesses of the surface being reduced to dimensions smaller 

 than the wave-length of light. Experiments on the thick- 



NO. 1858, VOL. 72] 



ness of glass removed by polishing and by etching with 

 hydrofluoric acid under various conditions were illustrated. 

 In the discussion Mr. Walter Rosenhain cited evidence to 

 show that the surface flow which has been recognised in 

 the polishing of metals also occurs in glass. 



Mr. Walter Rosenhain dealt with the possibilities of 

 progress in optical glass ; he described the limitations to 

 the production of vitreous fluxes of extreme properties, and 

 advanced the view that media of widely different optical 

 properties could only be obtained by the production of large 

 homogeneous crystals. Physicochemical considerations were 

 cited to indicate lines upon which this difficult problem 

 might be solved. 



A number of instruments for optical measurements were 

 described, Prof. Poynting exhibiting his form of parallel 

 plate micrometer. Mr. Blakesley described his apparatus 

 for the measurement of focal length of lenses, with appli- 

 cations to other optical measurements. Mr. Chalmers de- 

 scribed a new form of refractometer for obtaining the 

 refractive index of glass in lens form. The lens is inserted 

 in a trough containing a suitable transparent liquid, and 

 the difference of the refractive indices is deduced from the 

 approximate curvatures of the lens and its power in the 

 liquid, with an accuracy comparable with that of the best 

 refractometers. 



Mr. Baugh described the use of invar tapes for base- 

 line measurements. 



Dr. Drysdale discussed the requirements of small tele- 

 scopes and binoculars, with special reference to the field 

 of view and illumination of the image. He indicated the 

 method he had employed in calculations for prism 

 binoculars, showing how he had been led to use glass of 

 high refractive index for the prisms. He described a special 

 form of photometer for determining the absorption in 

 binoculars. 



Mr. A. C. JoUey gave a critical review of photometric 

 standards and apparatus ; he described a modification of 

 the VioUe platinum standard, and discussed the difficult 

 problems connected with heterochromatic photometry ; his 

 results indicate that the accuracy claimed by Sir W. Abney 

 is far too high, especially when readings by different 

 observers are compared. He concludes that a discrimin- 

 ation photometer is the most trustworthy instrument for 

 comparing different colours. 



Mr. Milne exhibited his new form of spectrophotometer. 

 The apparatus is especially suitable for determinations of 

 the absorption of light of specified wave-length by liquids. 



Mr. Bull discussed the theory of tricolour filters, plates, 

 and inks. He concluded that it was most satisfactory to 

 adjust each independently of errors in the adjustment of 

 the others. The filters should have a certain amount of 

 overlap, the colour of the overlap of two filters being the 

 colour of the printing ink corresponding to the other 

 filter. 



Mr. Crawley discussed the limits of stereoscopic vision ; 

 the results of his measurements point to a much greater 

 accuracy in judging distances by stereoscopic effect than 

 is generally admitted. 



Mr. H. L. Taylor discussed the effects of astigmatism 

 on the accommodation of the eye. Two new forms of 

 ophthalmometer were demonstrated, one being the Ettles- 

 Curties, which is valuable for the perfection of its 

 mechanical adjustments, and the use of complementary 

 colours for the mires ; it is so arranged that the corneal 

 microscope can be readily attached. The ophthalmometer 

 shown by Mr. Sutcliffe contains a number of variations 

 from ordinary forms ; the mire is an almost complete ring 

 illuminated by a special lamp, and the method of doubling 

 the image is novel. 



Dr. Walmsley gave an account of the attempts which 

 have been made to provide technical education for those 

 engaged in the optical industry, and the existing facilities; 

 he outlined the scheme for the establishment of a British 

 Institute of Technical Optics. The convention decided to 

 memorialise the London County Council to support the 

 scheme. 



Major-General Waterhouse gave an account of the history 

 of telephotography. 



In the evening "lecture. Prof. Silvanus P. Thompson gave 

 a most interesting account of the various forms of Nicol's 

 prism and its modern equivalents. 



