THE PRESENT CONDITION OF ENGLISH OBJECT-GLASSES. 267 



miniatured on the stage, when lit tip with direct svmlight, 

 have put me in possession of this general and important fact, 

 that as yet careful destruction of spherical'aberration deve- 

 lops colour in all the best glasses. Colonel Woodward, of 

 Washington, who has most handsomely placed at my dis- 

 posal a series of photographs taken by the actinic rays, of 

 monochromatic sunlight, has frequently confirmed my ori- 

 ginal statement as regards the colour test, as also in the last 

 number of the ' Lens' published at Chicago. 



The colour test was thus described by the writer : 



" I cannot too strongly call attention to the beautiful 

 phenomenon, which I have always endeavoured to obtain as 

 a fine and reliable test of approaching aplanatism, and herald- 

 ing a fine definition. In examining striated bodies, longitu- 

 dinal bands glisten with a ruby tint upon a green or yellowish 

 ground ; the bands appear like pellucid, semitransparent; 

 cylindrical ribs, and the flashing of these bodies with a ruby 

 glow is a signal, in my experience, that the aberration ap- 

 proaches a minimum ; when the beading dispels the mist 

 and haze always accompanying the spurious spines." — Page 

 302, ' Monthly Micro. Journal,' December, 1869. 



Agai7i, in the April number of the same journal, 1870, I 

 remarked : 



" I have lately seen the Formosum beading coloured with 

 red, orange yellow, and blue. The beading has appeared 

 wreathed with a golden bronzing ; individual beads, sepa- 

 rated from their fellows, appeared remarkably distinct. De- 

 struction of colour reduces the field to a spiritless picture. 

 Objects which can be, and in many cases are, lit up with a 

 startling brilliance of hue regain, with altered corrections, the 

 tame colouring of a dull prevailing yellow and black. There 

 is something recondite here beyond our ken.'' — Page 193. 



If we view diatoms in sunlight, e/i masse ,\)y t\ie unaided sight, 

 they appear to disperse their rays with a rich prismatic beauty, 

 the colours constantly varying with a change of position : 



" Yet these colours, so evident to the unassisted eye en 

 masse, vanish in achromatic vision; should these charming 

 colours be destroyed ?" — Page 193. 



These appearances are full of a significant interest ; still I 

 have constantly been able to see the most perfect definition 

 of minute markings only when I had disturbed the achroma- 

 tism insisted upon by the makers of the glasses. Whenever 

 I had so adjusted these glasses that the field was as perfectly 

 achromatic as possible, then the minutest structures formerly 

 visible, with colour, vanished. 



The conclusion to be deduced from these facts is this : that 



