140 THE AMERICAN MONTHLY [June, 



in dots, which has been ably disputed and may be still doubted 

 by some competent judges, is well within the limit of their ca- 

 pacity. The writer was fortunate in being afforded the rare 

 privilege of witnessing the official trial of the objectives by the 

 jury, at which time the Zeiss i-ioth (3.5 mm.) of n. a. 1.63, 

 with monochromatic sunlight illumination, showed the dots (or 

 " beads ") with beautiful distinctness and with perfect ease. They 

 were seen at a glance over a large part of the shell at once, always 

 alike and with remarkable freedom from any suspicion of uncer- 

 tainty ; and when lost by change of focus, or even by carrying 

 the instrument to another table and readjusting the light, they 

 could be recognized instantly, and every time, by simply correct- 

 ing the fine adjustment. As the most oblique pencils able to be 

 utilized by this objective, as employed, ought, according to the 

 Helmholtz theory, to be able to resolve lines of about 6,000 to the 

 millimeter, and as the A. pelhicida had about 3,700 rows of the 

 dots transversely, and 5,000 rows longitudinally, to the millimeter, 

 this resolution, while near enough to the theoretical capacity of 

 the lens to give an impressive demonstration of a near approach 

 to perfection in the construction, yet does not, by exceeding its 

 theoretical capacity, throw discredit upon either the genuineness 

 of the resolution or the accuracy of the optical theories involved. 

 But the durability of the apochromatics is, most unfortunately, 

 not equally plain. It is said that those first made were unsafe, 

 from the ease with which changes occurred in the frail phosphate 

 and borate glasses causing the dimness known as the disease of the 

 apochromatics. This was always freely corrected without charge 

 by the makers, and it is believed that the glass as now made is 

 free from this defect. The fluorite, however, must be taken with 

 its natural defects ; and its easy cleavage renders it very fragile. 

 In fact, it is now known that, more than thirty years ago, Charles 

 A. Spencer, of Canastota, N. Y., recognized its tempting optical 

 properties, and used it in the construction of objectives, one of 

 which is still in existence ; and that he abandoned the method 

 because of the early deterioration from the cracking of the spar. 

 That now used in the apochromatics is not only clearer and 

 remarkably free from color, but is evidently more durable, 

 and, it is hoped, practically permanent. The lenses are, however, 

 absolutely limited in output, as the world's supply of suitable 

 spar is confined to the stock now in hand, with no known means 

 of replenishing it. It is, therefore, none too soon to be finding 

 some method of making high-class work without it ; and this is 

 being attempted, with encouraging success, by several makers, in 

 the construction of the semi-apochromatics, made from the new 

 optical glasses, but without the fluorite. Though not quite as 

 free from color as the apochromatics, their views are very distinct, 

 and their working qualities extremely good. The resolution of 

 A. pellucida in " beads" by a semi-apochromatic is claimed to 

 have been accomplished in Italy. 



(Z'o be continued.) 



