150 Transactions of the 



judgment of mine would be premature. Last July I wrote to 

 Messrs. Powell and Lealand, requesting tliem to make one for the 

 Army Medical Museum, intending to try it in connection with the 

 immersion i^th of these makers. They were at the time, as I 

 understood, constructing one for Dr. Maddox, and I asked them, so 

 soon as that was finished to the satisfaction of Dr. Pigott, to make 

 the one for the Museum. This instrument has not yet reached me, 

 and from the information afforded by my London correspondent, I 

 fear I shall not receive it at any very early day. I am, however, 

 not unwilling to admit that demonstration alone could lead me to 

 believe that any optical combination introduced into the body of 

 the microscope, between the eye-piece and the objective, would 

 result otherwise than in loss of light and impaired definition. Espe- 

 cially should I anticipate this result, if the optical combination was 

 such as to increase the magnifying power. This is the result with 

 all the various achromatic ampUfiers I have experimented with, and 

 I have learned nothing as yet of the " Aplanatic Searcher " which 

 would lead me to suppose it escapes from the same defects. I 

 would suggest to Dr. Pigott a simple practical test that would go 

 far towards dispelling the doubts on this subject, which, so far as I 

 have been able to learn, are very generally entertained. This test 

 is afforded by an examination of Nobert's plate with and without 

 the Searcher. Can an objective which with eye-pieces alone just 

 resolves any given band on the test-plate be made to resolve the 

 next higher band by the Searcher? Will not its performance 

 rather fall to the next band below, or even farther ? It would also 

 be interesting to know whether the nineteenth band of Nobert's 

 plate can be resolved by any objective used with the " Aplanatic 

 Searcher." 



With regard to the Podura scale, it gives me great pleasure to 

 express the opinion that Dr. Pigott's observations constitute a 

 valuable addition to our knowledge of this test. On the coarser 

 Degeeria scale I have had no difiiculty in making out appearances 

 which, so far as I can gather from Dr. Pigott's own descriptions and 

 the pubhshed discussions with regard to his views, are substantially 

 the same as seen and shown by him. And even on the more minutely 

 marked and difficult Lepidocyrtus scale I have been able to develop 

 images which seem to be substantially similar. In arriving at these 

 results I have used chiefly the immersion |^th and ^Vth of Powell 

 and Lealand, with the ordinary eye-pieces. 



When Dr. Pigott's paper in the ' Monthly Microscopical Journal ' 

 of December, 1869, first reached me, my attention was especially 

 drawn to his novel views with regard to the Podura scale by the 

 personal allusion to myself on page 299. The paragraph is rather 

 indefinite, apparently owing to a misprint ; but I understand the 

 language used as intimating that it was chiefly the examination of 



