Limits of Microsco'pic Vision. By I)r. Royston-Pigott. 17 



each other, the smallest movement of the screw changes their 

 apparent thickness before division or separation is seen. 

 The miniatures were measured as follows : — 



At Distance. Miniature reduced. 



Inchps. Times. 



6| . . . . Very old i Powell and Lealand . . . . 49 



6 J .. .. 1862 i Powell and Lealand (immersion) .. 58 



61 .. .. 1875 A „ „ „ .. 55 



7 .. .. 1878 2^ R- and J. Beck (immersion) .. 140 



7 .. .. 1878 Jjj Beck (dry) 118-6 



6J .. ., 1873 ^ Gundlaeh (immersion) 91-3 



5 J ., .. 1877 i Zeiss (oil immersion) 49 



Qh .. .. 1S68 1-inch Powell and Lealand .. .. 6-07 



6i .. .. 1851 1 Andrew Boss 27-6 



6 .. .. 1870JWray 13-4 



6 .. .. 1870 1 „ 29-5 



To accurately adjust the observing and miniaturing objectives 

 in the same optical axis is easily done with low powers. If both are 

 equal in power, the test of the quality is very severe, as I have 

 shown elsewhere.* With a then excellent Powell and Lealand \ 

 made for me in 1862, and improved by them after its return to the 

 makers, a fog is still seen when observed by their brilliant newly- 

 formulated \ immersion. But still the spider lines are visible. It is 

 not till objectives of equal and I may say of surpassing beauty of defi- 

 nition are opposed to one another above and below, nose to nose, that 

 their exquisite powers of displaying fine black details are exhibited. 



The Gundlach immersion is of very fine quality. On reference 

 to the table, it diminished the spider lines 91 '3 times when the 

 distance between them and the miniature was 6^ inches. This 

 gave for the first and second lines (soW and yoVoth diameter 

 respectively) miniature sizes of 



^^*^^^ 73^°^"°^^'^'- 



2°^^*^^ 64^ " 



The sizes of the web No. 1 with the difierent objectives may 

 thus be tabulated : — 



Dist. Inches focus. 

 6i- 1 i Ross A Powell \ Powell and Lealand -Jjj Beck 



1 1 1 1 1 



48000 220000 300U00 460000 1120000 



These were mostly at 6^ or Q^ inches. At a greater distance 

 — 10 inches — the diameters of the spider hne of ^oVtt with the two 

 latter glasses would be, 



i Powell and Lealand -^^ Beck 



1 1 



640000 1600000 



* ' Phil. Transact.,' vol. ii., 1871. 

 VOL. II. • C 



