PROCEEDINGS OF THE SOCIETY. 727 



Mr. Edmund J. Spitta then gave a demonstration on " An Apparatus 

 for obtaining Monochromatic Light with a Mixed Jet." He commenced 

 by reminding the Fellows that resolution of minute structure depended 

 upon two functions, — the numerical aperture of the objective and the 

 wave-length of the light employed ; and since numerical aperture had 

 now reached its limit, at least so far as existing knowledge led us to 

 see, it was clear that any further advance for increasing resolution 

 must be looked for in the direction of the quality of the light, which 

 must be monochromatic and of the shortest possible wave-length. At- 

 tempts to obtain monochromatic blue light by means of glass screens 

 had failed beause blue glass of whatever quality he had tried always 

 allowed other colours also to pass, usually red, and so prevented the 

 light being purely monochromatic. 



Though this was considered by some as not so very important in 

 photography, if the plates used were not orthochromatic, still it was 

 sufficiently disturbing to prohibit the accuracy of focus for the demands 

 of study, when the Microscope was visually use-1. Screens consisting 

 of liquid solutions had also been tried, but though by this means it was 

 possible to obtain monochromatic blue light, it was found to be too faint 

 for use when employing a mixed jet. A portion of the spectrum from a 

 prism seemed more promising ; but this required the apparatus to be 

 arranged in a semicircular fashion and so rendered it unsuitable, in 

 addition to which the amount of light available was very small and 

 the beam very narrow — too narrow indeed to fill the field of a homo- 

 geneous one-twelfth. 



The diffraction spectrum reflected from a grating — a series of fine 

 lines ruled on a metal surface — was open also to the objection that the 

 light could not be used " direct." About four years ago, however, 

 Mr. Thorpe showed at the Eoyal Society's soiree some replicas off 

 diffraction gratings made by coating the metal with a thin film of 

 celloidin or some such substance, which when removed exhibited an 

 exact reproduction of the rulings, and consequently of the diffraction 

 colours, but with this very great advantage, viz. that the spectrum could, 

 be seen through the replica as it was so transparent. At that time it 

 was difficult to get a perfect film off a large enough grating to be of 

 any service for the subject in hand, but now Mr. Thorpe was able to 

 make them lj in. square. Moreover, he had lately achieved mounting 

 them on corrective glass prisms cut to the proper angle, so that any 

 single colour of special wave-length could be used " direct," which at 

 once did away with the objection of having to arrange the apparatus 

 in a semicircular fashion. 



Mr. Spitta then explained by a diagram on the screen how he 

 arranged his apparatus, pointing out the position of the collective con- 

 denser, the use of the collimator, and how the Microscope was placed. 

 He also spoke of the importance of using critical light and how it was 

 to be obtained. It was pointed out from the study of Abbe's law of 

 resolution to be an obvious conclusion, that as red light consisted of 

 about 30,000 waves to the inch and blue light of about 60,000, the 

 resolving power of an objective used with the latter would be just 

 double that obtainable by the use of the former wave-length. 



Three photographs of Ampliipleura pclhieida were then thrown on 



