438 Transactions of the Society. 



seen on the fine Navicula hjra (24,000), the Actinocyclus lialfsii 

 would be dotted, and the exclamation marks seen on a coarse 

 Podura (24,000). When an elementary form of condenser * was 

 used more could be done : striae could be seen on Pleurosiyma 

 formosum (25,000), Nilzschia scalaris (26,000), Doryphora Bocckii 

 (27,000). It will be noticed that with the apochromatic 24 mm. 

 objective, on account of its larger aperture, it matters a good deal 

 if a condenser is used or not, but with the lower-angled aa there 

 is not this difference in the image. It is for this reason that a 

 condenser is so seldom to be seen on an old Microscope. The 

 resolutions with these modern object-glasses were not pushed, they 

 were purposely used in a rough and ready way. It should be 

 specially noted that Dr. Smith's catadioptric had a potentiality 

 which other Microscopes did not possess. If, for instance, that 

 form of Microscope had been taken up, it would have been quite 

 easy to have computed and made others of greater aperture. There 

 can be no doubt that even in its original form it was by far the 

 best Microscope of pre-achromatic times, for, as we have seen, it 

 stood to other ancient Microscopes in the proportion of apo. 24 mm. 

 to semi-apo. aa. 



Finally, we come to the Wollaston doublets, which were invented 

 after the Microscope had been achromatised. The proboscis of a 

 blow-fly was examined with a single equiconvex, a doublet, and 

 a modern achromat, all of \ in. focus ; the doublet was just about 

 as much better than the single, as the achromat was better than 

 the doublet ; therefore, the doublet may be said to occupy a mean 

 position between the two. As the powers increase, the single lens 

 is rapidly left far behind. A doublet of 2 l s -m - focus will, when a 

 condenser is used, resolve the Pleurodynia formosum, a fine Navi- 

 cula lyra, a Nitzschia scalaris, and a strongly marked Podura 

 scale; a Chevalier doublet of the same power will resolve the 

 Doryphora Bocckii as well. The Wollaston doublet (made by 

 Powell) is a little sharper, but that by Chevalier is the stronger 

 resolver of the two. Without a condenser they show the Actino- 

 cyclus Ralfsii and the coarse Navicula lyra. So there is not much 

 to choose between these doublets and Smith's catadioptric Micro- 

 scope. 



We are now in a position to understand the true condition of 

 the microscopical world in pre-achromatic days. Smith's excellent 

 Microscope was practically unknown, and all that could be seen was 

 the best that a good compound Microscope of the Benjamin Martin 

 type could show. Just after the introduction of achromatism the 

 doublet was invented, which surpassed the Benjamin Martin 

 Microscope, and possibly would have equalled a Smith No. III. 

 if it had been made. 



In conclusion, we have some definite work which users of 

 * Dr. Smith's Microscope bad an equiconvex lens condenser. 



