6G8 SUMMARY OP CURRENT RESEARCHES RELATING TO 



the fluid-mounted slide, and using as a substage condenser a 2-inch 

 objective with the lower half of the front lens covered up, we have 

 been able to obtain with a minimum expenditure of time some of the 

 most satisfactory images by oblique light that we have yet seen. 



It may not be superfluous to remark that the most modern develop- 

 ments of high-power microscopy tend more and more to the prolonged 

 and exhaustive examination and study of individual specimens under 

 every variety of effects of light. Here again it must be acknowledged 

 that the swinging substage proves of service by the rapidity with 

 which the changes of illumination can be effected, and the certainty 

 with which they can be recorded and repeated. 



We do not here attempt to decide which special system of con- 

 struction is most effectual. The question whether the radial arc 

 originally devised by Grubb embodies the principle in the most prac- 

 tical form, is open to discussion, — and this applies equally to the 

 Thury-Nachet device, the Zentmayer's swinging arm, or Tolles's 

 traverse-lens. 



Some discussion took place at the June meeting of the Society 

 upon the preceding note, in which Mr. Crouch, Mr. Stephenson, and 

 others took part. 



Mr. Crouch said that in the early part of the year 1876 the 

 swinging substage was re-introduced by an American firm of opticians, 

 and exhibited at the Centennial Exhibition held in Philadelphia in 

 that year as an improvement upon the various forms then in use. " As 

 it was almost immediately adopted in this country, and has since, with 

 some slight modifications, been applied by various makers, both here 

 and in America, it has occurred to me that the time has arrived at 

 which it is possible to ascertain what the advantages offered are (if 

 any), and whether they are of sufficient importance to necessitate the 

 remodelling of those stands usually fitted with the substage. At any 

 rate, I think the discussion of this subject can only tend to the eluci- 

 dation of facts which are of imi)ortance in the construction of the 

 modern microscope-stand. 



" The substage, it need scarcely be stated, is an addition made to 

 the Microscope for the more ready application of the illuminating acces- 

 sories applied beneath the object, practically comprising the polari- 

 scope, various dark-ground illuminators, and the achromatic and other 

 condensers, either dry or on the immersion system. For the two first- 

 named methods it is not suggested that a swinging substage is of any 

 advantage. For immersion condensers, also, it cannot be of use, and 

 its application is therefore limited to dry condensers of presumably 

 small angle and comparatively long focus. Now, as the result of my 

 own experience, and also that of all those whom I have had the privi- 

 lege of consulting, this is found to be the most unsatisfactory and 

 uncertain of all methods of obtaining an oblique illuminating pencil. 



" It will at once be conceded that the proportion of objects for which 

 this method of illumination is useful is exceedingly small, consisting 

 mainly of a few of the diatomaceous tests, the resolution of the striation 

 of which requires objectives of great aperture and high power. The 

 immense majority of objects, however, shown by transmitted light 



