'264 SUMMARY OF CURRENT RESEARCHES RELATING TO 



shape. For ordinary use this front is covered with a brass cap, 

 having an aperture in the centre to allow the conical end of the 

 objective to pass through. The cap can be removed when it is desired 

 to use the objective for the examination of opaque objects. On 

 removal of the cap the conical sides of the lens are seen to be covered 

 with some sort of black varnish to prevent the passage of outside light. 

 A Lieberkuhn is furnished, which can be screwed on in place of the 

 cap while examining opaque objects. 



Scratching the Front Lenses of Homogeneous-immersion Ob- 

 jectives. — It was recently objected to homogeneous-immersion objec- 

 tives that the necessity of wiping the oil from the front lens after 

 each observation was fatal to their utility as in time the front surface 

 would thus become so scratched as to render the objective unfit for 

 use. 



This objection, however, overlooks the fact that even assuming 

 it was really impossible to properly clear off the immersion fluid 

 without " scratching " the lens, such scratches would not interfere 

 with the use of the objectives. As the fluid used for immersion is 

 homogeneous, that is, may practically be considered fluid crown glass, 

 the scratches are oj)tically obliterated as soon as they are in contact 

 with the oil or other medium ; in fact, it will be seen on reference to 

 the original paper of Mr. Stephenson on homogeneous-immersion 

 objectives,* that one advantage of the system was pointed out to be 

 that in petrographical work the very imperfect polishing of thin sec- 

 tions of minerals, which had previously been a source of difficulty, 

 was overcome by the approximately optical identity of the object and 

 immersion fluid. 



Fluids for Homogeneous Immersion.t — Dr. H. van Heurck, 

 Director of the Antwerp Botanical Gardens, has undertaken an ex- 

 tended investigation of fluids suitable for homogeneous immersion, 

 which (1) should have an index of 1 -510-1 -520 (line F), and (2) a 

 dispersive power of • 006 (between D and F), (3) should not be too 

 fluid, and (4) should not attack the varnish of the slides. 



Amongst the chemical solutions hitherto suggested. Dr. van 

 Heurck mentions Bassett's fluid (which attacks varnish), chloride of 

 cadmium in glycerine, iodide of zinc in glycerine, sulpho-carbolate 

 of zinc in glycerine, and distilled chloride of zinc (difficult to use and 

 not capable of being well preserved). Of the vegetable substances, 

 cedar oil and oil of copaiba are referred to. The first is a product 

 not of the cedar, but of Juniperus virginiana, and is much too fluid, and 

 attacks the varnish of the cells. The second (distilled from different 

 species of Dipterocarpus) is a little less fluid and therefore better. 



To remedy the inconvenience of the extreme fluidity of cedar-oil, 

 dammar has been dissolved in it, by which also its index may be raised 

 to I • 54. Professor Abbe has recently suggested to the author that an 

 excellent fluid may be obtained by dissolving dammar until the index 

 is 1 • 520, and then reducing it to 1 • 509 by the addition of castor-oil. 



* See this Journal, i. (1878) p. 52. 



t Bull. Sou. Belg. Micr., vii. (1881) pp. xxii.-xxxi. . 



