620 SUMMARY OF CURBENT RESEARCHES RELATING TO 



forward the opinion of so competent a judge as Dr. L. Dippel,* 

 against that of Prof. Merkel, who has objected to Merz's object- 

 glasses that they get dim from being too soft. Dr. Dippel writes : ' I 

 have lately become more closely acquainted with Merz's objectives, 

 1/3 1/9, 1/12, 1/18, and 1/24 in., and have convinced myself that 

 the' objection made to them by Prof. Merkel of their being affected 

 by the air is not well founded.' " 



Selection of a Series of Objectives.— At p. 449 (last line but 

 one) a misprint occurs of 200° instead of 120° as in Dr. Carpenter's 

 original text. 



Correction- Adjustment for Homogeneous-Immersion Objectives, t 



Dr. W. B. Carpenter's views on this somewhat vexed question 



are explained in his article "Microscope' in the 'Encyclopaedia 

 Britannica.' 



After pointing out that with homogeneous-immersion objectives 

 the microscopist can feel assured that he has such a view of his object 

 as only the most perfect correction of an air-objective can afford, 

 Dr. Carpenter continues as follows : " This is a matter of no small 

 importance, for while in looking at a known object the practised 

 microscopist can so adjust his air-objective to the thickness of its 

 cover-glass as to bring out its best performance, he cannot be sure, in 

 regard to an unknown object, what appearance it ought to present, 

 and may be led by improper cover-correction to an erroneous conception 

 of its structure. 



" It has been recently argued that, as the slightest variation in 

 the refractive index of either the immersion fluid or the cover-glass, 

 a change of eye-pieces, or the least alteration in the length of the 

 body — in a word, any circumstances differing in the slightest degree 

 from those under which the objective was corrected — must affect the 

 performance of homogeneous-immersion objectives of the highest 

 class, they should still be made adjustable. The truth of this con- 

 tention can, no doubt, be proved, not only theoretically, but practi- 

 cally, the introduction of the adjustment enabling an experienced 

 manipulator to attain the highest degree of perfection in the exhibition 

 of many mounted objects, which cannot be so well shown with 

 objectives in fixed settings. But it may well be questioned whether 

 it is likely to do the same service in the hands of an ordinary working 

 histologist, and whether the scientific investigator will not find it 

 preferable, when using these objectives, to accept what their maker 

 has fixed as their point of best performance. The principal source of 

 error in his employment of them lies in the thickness of the optical 

 section of the object ; for the rays proceeding from its deeper plane, 

 having to pass through a medium intervening between that plane and 

 the cover-glass, whose refractive and dispersive indices differ from 

 those of the glass and immersion fluid, cannot be brought to so accurate 

 a focus as those proceeding from the plane immediately beneath the 

 cover-glass. The remedy for this, however, seems to be rather in 



* ' Das Mikioskop,' 2nd ed., 1883, p. 460. 



t Encyclopsedia Britannica, 9th ed., xvi. (1883) p. 265. 



