38 PROCEEDINGS OF SOCIETIES. 



the hexagons to he the strongest parts, and led to the belief the 

 hexagonal markings on such diatoms were real ; but in the pleuro- 

 sigmas, and in many others, including several circular ones, the lines 

 of fracture passed between the bead rows, as long since shown by 

 Mr. Wenham. The structure of the hexagonal diatoms did not, how- 

 ever, vary in principle from the others, as there was evidence that the 

 whole floor of the hexagons was made up of minute spherules. He 

 hoped to publish some specimens of fracture where the hexagonal 

 markings were real, so that the Fellows might institute a comparison 

 between them and those which corresponded with simple beaded 

 structures. 



Mr. Gayer then read a paper " On a New Form of Micro-spectro- 

 scope." 



Mr. Wenham thought the arrangement described by Mr. Gayer 

 remedied one very great objection to the use of the ordinary form of 

 niicro-spectroscope. In the usual form the slit is placed in the eye- 

 piece, and the view is often drowned by diffraction lines. His impres- 

 sion was that putting the slit in the position indicated by Mr. Gayer 

 would obviate the objection alluded to altogether. 



Mr. Hogg said, there could be no doubt that the slit was placed 

 by Mr. Gayer in the proper position. But he thought Mr. Gayer 

 had laid too much stress on the dispersive power of his instrument. 

 It had been the object of Mr. Browning to get rid of dispersion : his 

 object was not to get dispersion, but a perfect image, with as little 

 dispersion as possible. It was not, as in the telescope, an object to 

 divide the sodium band, but it was a thing of great importance to get 

 a sharp, definite band under the microscope, and to be quite sure that 

 the substance under examination always gave bands in the same 

 part of the spectrum. Mr. Gayer said, that by using a deeper eye- 

 piece it was possible to remedy a defect in the instrument he was 

 using. He (Mr. Hogg) asked Mr. Gayer if he had examined a difficult 

 test, the chloride of calcium and cobalt, which was a very good 

 one ? 



Mr. Ingpen said, the form of Browning's spectroscope is that in 

 which the intermediate lenses are dispensed with, and the only thing 

 between the objective and the eye was the collimating lens. That gave 

 the sharpest spectra of anything he had ever seen. There was the 

 collimating lens just in front of a series of prisms which were viewed 

 through a small aperture in the other end of the instrument. No 

 eye-piece or intermediate lens of any kind was used. The whole in- 

 strument was put in instead of the eye-piece, after having focussed the 

 object with the ordinary eye-piece, and a sharp, beautiful short spec- 

 trum was obtained. Mr. Browning got rid of stray rays by slipping 

 a cap on the end of the objective, and then bringing that cap down so 

 as to touch the object. 



Mr. Slack would mention that the form desirable for a spectro- 

 scope depended necessarily on the purpose for which it was to be 

 used. The micro-spectroscope had been usually employed to examine 

 objects which give cloudy bands. It is a peculiarity of these bands 

 to be like astronomical nebuke in the telescope. Consequently, if 



