Royal Microscopical Society. 153 



III. — Note on the President's RemarJcs on The Searcher for 

 Aj^lanatic Images, as to the Principles upon which it acts. 



By Dr. Eoyston-Pigott, F.E.S. 



(^Read hefore the Eoyal Microscopical Society, March 4, 187-i.) 



I REGRET that the paper in the ' Philosophical Transactions ' on this 

 suhject has not heen printed in the Journal, as also the two quarto 

 plates. (I have placed fifty copies at the disposal of the Society.) 



The searcher gives a new means of balancing spherical and 

 chromatic aberrations. 



It is on a large scale precisely what the adjusting screw- collar 

 of an objective is on a minute scale. The collar separates the front 

 lens by thousandths of an inch. The searcher traverses inches. 



Compensating lenses may be applied at many different places. 

 Thus, Mr. Wenham's improved object-glass depends principally 

 upon the compensation of the back lens or posterior glass of the 

 combination for balancing the aberration introduced by the front 

 sets. If he were to make this back lens traversing on the principle 

 of the searcher, he would have a wider choice of balancings. 



The thick front introduces very considerable aberration ; this is 

 corrected more or less perfectly by the posterior lens. 



Now the aberration of a lens varies rapidly with the diameter 

 of the aperture. If the searcher be placed in a more distant 

 position, a smaller pencil engages its surface, and its aberration is 

 diminished. 



If a short tube be used — say 5 inches instead of 10 — the object- 

 glass becomes violently under-corrected. An over-corrected lens, 

 properly chosen, may be found to correct this. But as it is impos- 

 sible practically to apply an infinite number of differently corrected 

 lenses, the more simple expedient of placing a given over-corrected 

 lens nearer or farther from the objective answers precisely the same 

 purpose. 



By patiently moving the searcher into different positions, and 

 also altering the screw-collar, some particular position is realized 

 which just balances the objective residuary error in the most 

 satisfactory manner of which the workmanship of the objective is 

 capable. 



In doing this, miniatures of known objects formed by a first- 

 class objective are the most satisfactory as a thermometer bulb or 

 watch-dial, or mercury globules of the size of large shot illuminated 

 by the sun and placed at 10 inches below the stage,* form jaagni- 

 ficent tests, and obviate the difficulties attending side illumination 

 on the stage of globules of the smallest kind; a good objective 



* See 'Phil. Trans.' 



