270 MEMORANDA. 



Moreover, some persons do not use a lower power than 

 1-inch; and, if so, and that lens be employed as the "finder" 

 with eye-piece d, I do not believe there is any organic object 

 that cannot be distinctly discerned thereby; as that com- 

 bination will clearly show a particle sufficiently small to 

 pass through the mesh in the above-mentioned scale (taken 

 from the under side of the wing), which meshes I find, by 

 micrometrical measurement, to average about y^^th of an 

 inch in length and xg.oVo^ 1 m breadth.* 



This, as formerly said, I will undertake to prove to any 

 one who may think it worth while to come here for the 

 purpose. 



So that, to shorten the matter as much as possible, the 

 entire question may be resolved into the following heads : 



1st. The double nose-piece is amply sufficient for the im- 

 mediate finding, with the highest powers, of all the generality 

 of objects, even those which are totally invisible to the un- 

 assisted eye. 



2d. Those objects that are difficult of detection with so 

 low a power as 1^-inch are (comparatively) very limited in 

 number ; and that any of them are impracticable (with eye- 

 piece d) I have great doubt. 



3d. Eyesight differs greatly in quality in different people ; 

 so that mieroscopists must be cautious in pronouncing " that 

 will never do," &c, when the whole truth is, that will never 

 do for me. 



For my own part I am more and more pleased with the 

 1 ' nose-piece finder," and am using it continually, to my 

 very great comfort ; but for the benefit of those who arc not 

 satisfied with it, and are especially bent on screwing their 

 unfortunate optic nerves to hunt out those excruciatingly 

 small objects which they cannot find with inch objective and 

 eye-piece d, it may be well to state that of all the finders 

 hitherto devised on the graduated-plate system, Mr. Kitton 

 gives the preference to that of Mr. Maltwood, described in 

 your journal for April, 1858. 



In conclusion, my remarks have been given upon the 

 principle of " valeat quantum ra/ere potest/' and my readers 

 will now, of course, do as they please in the matter; but I 

 cannot refrain from giving them a well-meant caution, that 

 the less they strain their " visual optics " over these almost 



* These measurements were made by means of the pood old stage-micro- 

 meter, with which .Mr. Powell used to supply his instruments many years 



ago. I think highly of it, and do not know why it is now so rarely made. 

 One turn of the wheel is the jfoth of an inch; aud the wheel being divided 

 into 100 degrees, one degree equals -rckroth. 



