66 The Advancing Powers of Microscopic Definition. 



notice more in detail the performance of Messrs. Powell aud 

 Lealand's new 1-1 6th objective, which they kindly placed at my dis- 

 posal for a fortnight. This lens is similar though perhaps slightly 

 superior to the celebrated immersion 1-1 6th signalized by Dr. 

 Woodward as the only one which resolved Nobert's 19 th band so as 

 to be counted, and photographed the Amphipleura pellucida. 



As I had announced in May, 1869,* a double row of 

 spherules in diatoms (when only one-half had been suspected), I 

 particularly wished to confirm this appearance. It is well known 

 that formerly black dots used to be seen above the true beading 

 (well figured by Beck). The new 1-1 6th showed exactly double 

 the number of these dots. In individual spherules of the For- 

 mosum, no less than three different colours could be descried upon 

 each boss or bead, and with oblique achromatic light a black 

 shadow in addition. The same appearances resulted from a skilful 



use of their new -^th immersion. 



I here beg leave to insert the 



notes taken May 25, 1871, and dictated by an observer with good 

 powers of vision, and confirmed by myself. 



^^Particulars. — Condenser ll-in. Eoss, obliquity of its axis 30^. No 



stops or diaphragm. Powell and Lealand's best 1-1 6th immersion. 



" (1) White heads with red tops, and green narrow parallel stripes 

 in one direction only between the beads. 



" (2) A rather deeper focus, white heads and red heads ; fainter 

 stripes between : the dots form diamond patterns. 



" (3) Focus deeper still. Prominent white beads ; black shadow 

 on the left and on bottom of the bead (lower side). There is a 

 reddish background, the right-hand side of the bead is green. 



" (4) Obliquity of axis of condenser reduced to 10°. Green and 

 white beads, alternate rows, most distinct without any background 

 whatever, the whole space being entirely covered with closely-packed 

 beading, so that the white set of beads appear in the blank spaces 

 figured in the American photographs." 



These are apparently exactly twice the general quantity of beads 

 usually seen, the blank spaces teeming with them. When the 

 obliquity was reduced to zero, or the axis of the 1^ condenser 

 (unstopped) coincident with that of the instrument, the new beads 

 came out beautifully distinct under the most careful corrections, as 

 green, black, and white, as follows, representing them by G, B, and W. 



G 



w 



w 



w 



w 



w 



G 



B 



G 



B 



G 



B 



Green, Black, Green, Black, forming rows with white between." 

 * Paper received May 21, 1869. 



