247 



On Arachnoidiscus as a New Test for High Power 



Objectives. 



By T. F. Smith. 



(Bead March 23rd, 1888.) 



For optical appliances this is the golden age of microscopy, and 

 it may be confidently asserted that a better object-glass can be 

 obtained to-day for five guineas than could have been bought only 

 a dozen years ago for fifty. In view of the fact that the same 

 opticians who were to the front as makers of objectives twelve years 

 ago are to the front now, this may seem a bold assertion, until it is 

 remembered that it is only ten years since the first oil-immersion 

 was made, and not two years since the new optical glass has been 

 in the market. Both these occurrences are epoch-making, and as 

 an instrument of optical precision the Apochromatic oil-immersion 

 object-glass of to-day is almost as much separated from the best 

 work of twelve years ago as the Achromatics of, say, 1850 were from 

 the old Non-achromatics of the early part of this century. Having 

 come into this splendid possession, then, it only remains to know 

 how best to use it to advantage, and I beg to-night to offer you 

 my experiences during the last few months as a contribution to that 

 knowledge. It will be in the memory of the Members of this Club 

 that I have from time to time exhibited objects in this room to 

 illustrate some papers read by me, and that, having sometimes 

 exhibited three or four objects at once, I have had to borrow glasses, 

 in addition to my own, to show them. It is also vividly within my 

 own memory that when I borrowed four oil-immersion lenses, in 

 fixed settings, for the purpose of my first paper, and tried them on 

 the objects I knew so well, they utterly failed to show them in a 

 satisfactory manner. It was not that I could not see the details, 

 but the image was dull as ditchwater, and wanting in that lumin- 

 osity which an oil-immersion should always display. My first feel- 

 ing was one of disgust at my want of success, and a desire to throw 

 the thing up as a failure ; but a few trials convinced me that the 

 cause of the mischief was the lenses being set at the wrong point, 



