81 G SUMMARY OF CURRENT RESEARCHES RELATING TO 



opticians will tell you that they know the best point, but bis cxporionco 

 is as follows : — 



Four object-glasses, with a correction-collar, were sujiposcd to bo 

 set with best definition on the Poduia scale at tli(! point ; the first was 

 best on a balsam-mounted slide at point 2}j ; No. 2 glass was at its best 

 ftt point 5 ; No. 3 at point 7^ ; and the last glass at its best on the same 

 slide at point 10, or as far as it could go. It is no use blaming opticians, 

 for the English microscopists have been brouglit uj) (and rightly, up to 

 a certain point) to believe in the Podura scale, and makers cannot be 

 expected to rim the risk of producing a glnss that is not at its best on 

 that test. The only way then is to oHer a substitute that shall stand for 

 the oil-immersion in the same relation as the Podura scale does to tho 

 dry glass, and for that purpose Mr. Smith " offers the outer plate of the 

 Arachnoidiscus (anything) mounted in balsam." 



To him there is a i)articular appropriateness in choosing this as a test 

 object, from the fact that although its main features for the last forty 

 years have been as well known as tlie Podura scale itself, the discovery 

 of the finer markings or structure is due entirely to the oil-immersion 

 objective. 



The advantages claimed for the new test-object for an oil-immersion 

 are that the little projecting points or si^ines can only be clearly 

 defined where the objective is perfectly corrected and set at its best point. 



It is not every disc of the diatom that will act as a test, any more 

 than will every scale of Podura. Some will show no projecting spines 

 even with the widest-angled objective, and others are so coarse as to bo 

 no test at all ; but a properly selected one will answer all the purpose, 

 both for defining and resolving power. 



Tests for Modern Objectives.* — Mr. E. M. Nelson considers that the 

 advance of the Microscope in recent years is due to the Podura scale and 

 the following diatoms : — 1st, Bhomhoides ; 2ndly, Grammalophura suhti- 

 lissima ; 3rdly, and probably to a greater extent, Amphipleura peUucidn ; 

 lastly, and at the present time, to Pleiirosigma angulatum, N. rhomhoides, 

 and the secondary markings of diatoms in general with large angled 

 cones of central light. It was the demand for glasses that would give 

 classical images of the Podura scale which improved the central portions 

 of the objectives, and it was the demand for diatom-resolving lenses 

 which spurred on the opticians to make wide angles and to correct the 

 margins. 



But however much we may regret it, these old tests — the Podura and 

 the Amijliipleura pellucida — which have been of great service to the cause 

 of microscoi>y, must be laid aside. The classical picture of Podura 

 demands such a very small area of the centre of an objective that it tests 

 too little of the glass. 



The following are a few tests for modern objectives : — 



1. Pleurosigma angulatum, showing dark perforations on a light 

 ground, with a fracture passing through them. While the dioptric beam 

 passes though the centre of the lens the diffraction spectra sweep the 

 margin. Unless a lens be truly centered it will not stand this test. 



2. A Cherryfield Bhomhoides in balsam or styrax with the full aper- 

 ture of Powell's latest condenser is a very severe test. 



* Engl. Mcch., xlviii. (18S8) p. 51. 



