1897] MICROSCOPICAL JOURNAL. 85 



The following" table shows what has actually been 

 achieved on diatoms in balsam with a 'i axial cone. A 

 comparison of this table with the former will be instruc- 

 tive: — 



Table II. 

 Objective. N. A. Resolved. 



Apochromaticlin 32 '22,000 



Adiromatie 4-10 64 40,000 



Apochromatic .V -66 46,000 



Semi-apochroiuatic 1 "71 53,500 



Achromatic 1 -79 53,000 



Serai-apochromatic 1-7 '86 60,000 



Achromatic 1-5 -88 60,000 



Apochromatic 1 -95 65,000 



Semi-apochroraatic 1-12 1-26 90,000 



Apochromatic i 1*43 94,000 



I do not know which of the Coscinodisci has the coars- 

 est secondaries. Asteromphahis is fairly coarse. Some 

 of the THceratia have very coarse secondaries — Thumii 

 may be one of them. 



With regard to mounting media, there has been too 

 much made of high refractive index, and too little of 

 spectrum irrationality. Piperiue has a high index, but 

 its irrationality spoils it. 



The order of merit may be taken as follows: — 



1. (^uinidine, by far the best; unstable. 



2. Styrax, very good and permanent. 



3. Balsam, permanent. 



4. Monobromide, not good. 



Prof. Smith's, Dr. Meale's and Father Thompsan's 

 media are uncertain, difficult, and very dangerous to work 

 with. 



In conclusion, let me urge workers to procure a (Tif- 

 ford's F line screen for use at the back of the condenser; 

 they are quite inexpensive. They greatly improve the 

 definition, and make cheap semi-apochromats almost 

 equal to the most expensive apochromats; they even im- 

 prove apochromats, and they increase the resolving 

 power. — English Mechanic. 



