262 DIl, ROYSTON-PIGOTT. 



miniaturelandscape within the drop of water, or even glass rods 

 crossed, forming distorting lenses and curious images when 

 illuminated from behind, or a thousand other contrivances, 

 may all be used to display the state of the magnified miniature. 



In this manner I had the good fortune to detect a strong 

 yellow fog in a J^th objective^ — pronounced very fine by' 

 Messrs. Powell and Lealand — which had escaped their detec- 

 tion by the ordinary tests employed by those excellent makers, 

 now unsurpassed throughout America and the continent. 



The very trying and fatiguing labours of these makers to 

 diminish the residuary errors of their glasses, so soon as 

 pointed out, have been rewarded with unprecedented suc- 

 cess. The introduction of a saturated solution of the am- 

 monio-sulphate of copper, as destructive of the red and 

 yellow rays of the solar spectrum, and producing a blue ray 

 sufficiently monochromatic to eliminate the aberration of the 

 yellow-red rays, has been recently crowned with a most gra- 

 tifying success. Under these circumstances, what is so diffi- 

 cult to see with compound light is photographed actinically 

 with an unaberrating ray. I had the honour of exhibiting, 

 at the soiree of the Royal Society this year, a series of photo- 

 graphs presented to me by Colonel Dr. Woodward, in which 

 the T^th innnersion had depicted, by means of the blue ray, 

 the beaded structure of the Podura Degeeria and P. curvi- 

 collis test-scale in the greatest perfection. This glass is the 

 only one that ever succeeded in photographing Nobert^s 

 band 19, containing 112,000 lines per inch. These results 

 teach us that to see an object in its purity and integrity under 

 very high powers requires the use of contrivances for eliminat- 

 ing the residuary aberration. This is done by Colonel Wood- 

 ward's plan to a certain extent, and also by the *' searcher 

 for aplanatic images " much used by the writer. 



But there is another point which is a very rife cause of 

 disagreement amongst observers while using high powers, 

 viz. what is the true focal plane of vision ? one professing 

 this and another that. A most striking example of this is that 

 the Podura markings, as usually seen, are shadows produced 

 by the crossing of two different sets of structures on different 

 planes, and that the upper beading is seen on a plane above 

 what is usually focussed for. 



Another cause of wrong observation and interpretation is, 

 the observer and maker spherically under-correcting the ob- 

 jective at the time of observation. Messrs. Powell and Lea- 

 land, for instance, frankly admit that they cannot see the 

 Podura markings well on their best scales, except their 

 ' ' PbU. Trans,' vol. ii, 1870, pages 592-3. 



