1897] MICROSCOPICAL JOURNAL ,S3 



The following;- table shows what lius actually been 

 achieved on diatoms in halsain witli a ■{ axial cone. A 

 comparison of tliis table with t.lie former will l)e instruc- 

 tive:— 



Tahi.k J I. 



Olijective. N. A. Ivesolved. 



Apochroiiiatic lin -32 '22,000 



Achromatic 1-10 •(i4 40,000 



Apochromatii' .] -fiC 4G,000 



Soiui-apinliroiuiitic I "71 ^S.SOO 



Athroinatic I -79 53,000 



Semiapochromatio 1-7 •iSC 00,000 



Achronjatic 1-.') -stt 00,000 



Apochroniatic ', •!»;"> 65,000 



Seini-apochromatic l-l'i \'26 90,000 



Apochroiiiatic i 1 •4:? 04,00(1 



1 do not know which of tlie Coscinodisci has the coars- 

 est secondaries. Asteroviphnlus is fairly (;oarse. Some 

 of the Trice rat la liave very coarse secondaries — TJttrniii 

 may be one of them. 



With reji;ard to numnting media, there has been too 

 much made of high refractive index, and too little of 

 spectrum irrationality. Pi[»erine has a high index, but 

 its irrationality spoils it. 



The order of merit may be taken as follows: — 



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



2. Styrax, very good and [lermanent. 

 'A. Balsam, permanent. 



4. Monobromide, not good. 



Prof. Smith's, Dr. IVIeale's and Father Thompson's 

 media are uncertain, difficult, aiul \ ery dangei'ous to work 

 with. 



In conclusion, let me urge workers to ])rocure a (iif- 

 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 a|»ochromats: they even im- 

 pr<n'e apochromats, and they increase the I'esolvino' 

 power. — English Mechanic. 



