218 Dr Royston-Pigott, On a new method of [Dec. 2, 



interval between the front set of lenses, which gives the posterior 

 lenses greater or less power of correcting the aberrations, which 

 change considerably if the distances between object and image 

 vary. 



Thus, supposing a tube, used below for giving the miniature, 

 is so short for convenience, that there is only one interval of four 

 and a /ia// inches between object and image, the miniature formed 

 (if the glass is adjusted for ten inches) will be found to be 

 violently " under co7'rected/' and in order to get a fine definition 

 in the miniature the interval between the front sets of glasses 

 must be gradually increased by the screw collar until the defini- 

 tion assumes a fine character, resembling the view displayed by 

 a good opera-glass ; whilst bad definition will resemble the effect 

 seen through the opera-glass in a London fog by gas-light. 



The same precaution is requisite for observing with the micro- 

 scope itself. In some cases the finest results are obtained with 

 " Immersion lenses," and I have found in a solution of the chloride 

 of gold^ in glycerine a very beautiful sharpness of definition. 



The precise details for managing these arrangements are given 

 in the Phil. Trans, of the Royal Society and in the Proceedings, 

 especially the latter, which elaborates the matter in a paper on 

 " Circular Solar Spectra," obtained by these means, respecting 

 which it may be permitted to remark here, that the diameter of 

 spurious disks was measured in the microscope: as formed by 

 miniature of the sun's orb examined under a power of one thousand 

 diameters on the stage of the microscope. In this arrangement 

 tihe whole series of glasses combined in giving the final image, 

 formed at the focus of the eye lens, in a manner precisely similar 

 in all respects to the method now described. But in these elabo- 

 rated experiments, long continued, the most remarkable pheno- 

 menon observed was this : During the sunshine, every bright 

 object line or spot was blurred and obscured, the distant dazzling 

 aerial image of the sun being shaded, but the moment a cloud 

 passed over the sun every minute object in the miniature land- 

 scape became perfectly distinct and sharp. 



Just the same happens in the system of lenses as now arranged; 

 brilliant lines and disks are enlarged spuriously and obscure dark 

 lines. But dark spider-lines, free from brilliant diffractions, can 

 be beautifully defined on a white ground. The celebrated optical 

 law, deduced independently , by Professor Helmholtz, F.B.S., and 

 also by Professor Abbe of Jena (alluded to further on), receives 

 a beautiful illustration by the experiments in the papers cited. 

 The solar disk measured, after its miniature had been effected (1138 

 divisions of the micrometer being equal to one thousandth of an 

 inch on the stage), nearly jewo ^^ ^^ 'mch. ; whilst the breadth of 

 ^ 2 grains to the draebm. 



