147 



VII. — On the Visibility of the Tertiaries of Coscinodiscus 

 asteromphahis in a Balsam Mount. 



By Edward M. Nelson. 



(Read March 16, 1910.) 



Some twelve years ago, when examining a slide of Nottingham 

 deposit, mounted in sulphide of arsenic by Dr. Meates, and kindly 

 presented to me by Mr. Ingpen, I saw for the first time the ter- 

 tiaries in the Coscinodiscus asteromphalus. These tertiaries are 

 formed by a sieve covering the dots surrounding the primary areo- 

 lations of this diatom.* 



A balsam-mounted selected slide of this diatom has been in my 

 box of test-objects since 1876, and it is no exaggeration to say that 

 during that time hundreds of objectives have been tested upon it. 



When an objective is tested, an endeavour is made, by means of 

 a large axial cone and deep eye-piecing, to obtain the very utmost 

 that can be got out of the lens, and therefore if these tertiaries had 

 been visible in this balsam mount such an important fact would 

 have been noted. 



Towards the end of last month a long tube -^ apochromatic 

 N.A 1 '4 was received from Messrs. Zeiss, and in being put through 

 its routine of tests an examination of this old balsam-mounted 

 specimen was made. The tertiaries which had for so many years 

 eluded the grip of all kinds of lenses were conspicuous. 



Here, then, we have a definite case : the specimen, the Micro- 

 scope, the substage condenser,* the lamp, the method of work, and 

 the eye at the eye-end (hardly improved for being twelve years 

 older) were all the same. The only possible explanation why 

 detail formerly invisible should without any particular difficulty 

 now be seen, is that there has been an improvement in the objective. 



Microscopists frequently assign improvements in the Microscope 

 objective to definite epochs ; for example, the date of its achroma- 

 tisation, or the date of its apochromatisation. It is true that at 

 these dates the capacity of the objective went up several steps at 

 one bound; but it is also true that several single steps — and shall 

 we say half-steps — were made at other times that were hardly, if 

 ever, noticed by the user of the Microscope. 



Some have said that the Microscope objective was apocfaro- 



• Journ. Quekett Micr. Club, vii. (1898) p. 81, pi. 8, fig. 8. 

 t See this Journal, 1895. p. 229, fig. 32. 



