On Mounting " Selected " Diatoms on Slip. 

 By H. Morland. 



{Read December ISth, 1891.) 



In a paper on mounting the Diatomaceae whicli I read before 

 the Members of this Clnb some three years ago, I there stated 

 that it was my practice to mount "selected" diatoms on the 

 underside of cover-glass, and that far too often I found the gum 

 by which the diatoms were fixed showing up in a most objec- 

 tionable and unpleasant manner when the mount came to be 

 viewed under the microscope. So long as diatoms are fixed on 

 the underside of cover- glass I see no way of avoiding this 

 mishap with any degree of certainty, particularly if the diatoms 

 be mounted with the outer surface next to the gum or other 

 fixing cement. 



In the old days of high power " dry " objectives it was a 

 matter of absolute necessity to have the diatoms close to the 

 cover by reason of the extremely short working focus of these 

 objectives, but now that homogeneous immersion lenses are 

 almost universally employed for the higher powers, and as 

 these lenses have a considerably greater working distance, this 

 absolute necessity of the diatoms being in actual contact with 

 the cover-glass may be said to no longer exist, at least, not in 

 999 cases out of a thousand ; all that is necessary is to have the 

 diatom well within the focus of the objectives. 



Messrs. Zeiss and Co. in their pamphlet, when introducing 

 the apochromatic lenses to the public, claimed that their highest 

 power lens with their highest IS'.A. (viz., the lens of 2 m.m. and 

 1*40 N.A.) will work through a cover-glass of '25 m.ni. (or y^ 

 of an inch) thickness. By far the larger proportion of homo- 

 geneous lenses have ample working distance, and so long as I 

 have -pJ-jY of an inch at my disjDOsal I have sufiicient room, and 

 to spare, in which to mount my diatoms direct on to the slip 

 instead of on to the cover. 



The advantage of mounting on the slip is that as most 



