WALTER ROSENHAIN 131 



to the use of eye-pieces such as the compensating eye-piece of Zeiss, 

 which can only be used with a particular series of objectives. Unless, 

 therefore, such an arrangement is really essential to allow the best 

 results to be obtained, it will be very much preferable to have 

 eye-piec€g and objectives self-contained and interchangeable, not only 

 with other lenses of the same series, but as nearly as may be 

 universally. It may be desirable to state from the author's practice 

 and experience the most useful focal lengths for objectives and 

 magnifications for eye-pieces. It should perhaps be said that it is 

 not suggested that any rigid standardisation of magnifications should 

 be adopted by metallurgists. While a certain degree of uniformity 

 of practice and especially the avoidance of odd magnifications are 

 no doubt desirable, any attempt to tie down microscopists to a few 

 specified magnifications is eminently undesirable, since the magnifica- 

 tion for each subject should be chosen specifically to suit that subject. 

 A range of objectives and eye-pieces is, therefore, in the author's 

 opinion, desirable, which will allow of almost any desired magnifica- 

 tions being obtained in a satisfactory manner, that is, by use of an 

 objective of adequately resolving power and without employing a 

 high eye-piece or an unduly extended camera, where photographs 

 are concerned. 



The lenses ordinarily used by the author have focal lengths of: — 

 16 mm. I 

 8 mm. rdry series. 

 4 mm. ) 



2 mm. , ., . 



o oil immersion. 



3 mm. I 



Eye-pieces — x 8, x 12, x 18. 



These lenses have been used because they have been commercially 

 available in those makes which have in the past produced the finest 

 results. So far as the objectives of the dry series are concerned, 

 the focal lengths stated fulfil all ordinary requirements, although 

 a 4 mm. dry objective is not easy to use and requires a great deal 

 of stopping down of the beam of incident light. For this reason, 

 the author, some time ago, suggested the desirability of an immersion 

 lens of from 5 to 7 mm. focus. This would have the great advantage 

 of affording a greater depth of focus than the 4 mm. dry objective, 

 but it might prove difficult to use in a horizontal position unless a 

 special device were provided for holding the oil in place. 



With regard to the immersion objectives, that which has given 

 the finest results for the highest magnifications in the author's 

 practice, has been a lens of 3 mm. focus with N.A. 1.40. Unfor- 

 tunately, these lenses are very delicate in use, and require not only 

 protection from mechanical injury, but also from any agency which 

 affects the cement with which the front lens is attached to the mount 

 and from prolonged exposure to contact with immersion oil. If 

 the latter is not of precisely the right quality, this is alone sufficient 

 to do damage. If this oil is wiped away very gently with a soft cloth 

 and the surface of the lens is then wiped lightly with an old hand- 

 kerchief slightly moistened with benzol, damage to the cement may 

 be avoided for a long time. 



