A German Silver Powell Portable Microscope. 169 



a clear image magnified up to 2000 diameters by deep eye- 

 piecing, but the ^ in. * attained the mark in a different manner, 

 aud thus placed the old workers almost on an equality with their 

 successors of the present day, so far as regards the optical 

 capabilities of dry lenses. It is also a fact that the practical 

 manufacture of a good objective of 0*91 N.A., with such a high 

 initial magnification as 185 requires very great technical skill in 

 consequence of the smallness of its component lenses, and it is 

 doubtless owing to the unrivalled workmanship of Powell that the 

 corrections of these abnormally high power objectives were 

 brought to such a state of perfection as to engender the idea that 

 power per se was necessary and the goal to be attained, so that 

 subsequently we find the same firm constructing dry ^ in. and 

 gL i n- objectives of but little higher angle than the -fa in. 



With reference to the origin of this once prevalent idea, the 

 writer has very carefully compared the performance of the |- in. 

 •of - 89 N.A. and the ^ in. of 0-91 N.A., using in both cases the 

 full axial cone of the old condenser. The P. angulatum was 

 chosen as a test object, the general strength and quality of the 

 image being taken as a criterion. With merely a signal green 

 glass modifier the performance of the \ in. proved highly satis- 

 factory, and decidedly better than that of some modern lenses 

 of considerable pretensions which have come under notice as 

 claiming to be very superior and up-to-date productions. On 

 substituting the -^ in., the resolution and quality of the image 

 appeared strengthened, and the general effect was improved more 

 than the slight increase in aperture would warrant. 



This gain can only be attributed to the superior correction of 

 the ^ in. over the undoubtedly good \ in., but one can easily 

 imagine how the old observers were misled by it, and decided, 

 erroneously as we now know, that power per se in the objective 

 was an advantage, and the object to be aimed at. Hence the 

 demand by investigators for dry objectives of ^5, -£$, and even 

 gL in. foci, all of about similar N.A. to the ^ in., which one 

 would imagine had already a sufficient ratio of power to aperture, 

 and the corrections of which could not be much improved upon 

 owing to the increasing technical difficulties encountered in the 

 construction of smaller combinations^ An idea once originated, 

 and generally accepted, is, however, hard to kill, and we conse- 

 quently find that the envied possessors of the costly -^ and 

 Jo in- objectives were for long fully satisfied regarding their 

 optical efficiency. 



* Powell first constructed a ^ in. in 1840. 



April 21st, 1909 N 



