366 PROCEEDINGS OF THE SOCIETY. 



Professor Abbe's Note on a fluid for homogeneous immersion was 

 read by Mr. Stephenson, who said he had just received a letter from 

 Professor Abbe, informing him that his assistant, Dr. Riedel, had 

 lately found two fluids suitable for homogeneous-immersion objectives. 



(1) The first is a solution of gum dammar, dissolved in hot oil of 

 cedar-wood. The oil which is obtainable in Germany has a refractive 

 index of 1 • 51 onhj, but by the dammar this can be raised to 1 • 54. 

 This solution is, however, rather highly coloured, and of course 

 somewhat sticky. But if it is carefully distilled, it becomes sufficiently 

 pale and loses its stickiness. It appears that the distillation causes 

 a certain molecular change or decomposition of the resin, by which its 

 stickiness is destroyed. By diluting the strong solution (which con- 

 tains a small portion only of the resin) with pure cedar-oil, every 

 index from 1*51 to 1"53 can readily be obtained. Professor Abbe 

 takes 1*52 exactly as his standard, at a temperature of 18° Cent. It 

 is unchangeable, and, like pui'e cedar-oil, does not act upon sealing- 

 wax, or shellac, varnish. 



A very important point, to which he (Mr. Stephenson) had pre- 

 viously referred in that room, is the necessity of determining the 

 dispersive power of all fluids used for this purpose ; and with respect 

 to this, Professor Abbe remarks that, although other resins might be 

 used in a similar way, all those which he has tried give too large an 

 increase of dispersive power, which is not the case with dammar, and 

 consequently the new fluid described by him is in every respect very 

 near to fluid crown. 



(2) The other medium is a solution of iodate of zinc in Price's 

 ordinary glycerine (n = 1'46). This salt is very soluble in glycerine, 

 and a refractive index of 1"56 or more can be readily obtained, and 

 there is therefore no difficulty in making a solution of 1 • 52, which is 

 the standard index at 18^ Cent. Like glycerine, it is of coiu'se 

 hygroscopic, and its use therefore requires some care ; at the same 

 time. Professor Abbe states that it is as inaggressive as the solution 

 of sulpho-carbolate of zinc or of chlorate of cadmium, whilst it is 

 far less sticky ; provided the refractive index has really been raised 

 to 1-52 only. Its dispersive power is slightly in excess of the 

 solution of dammar in cedar-oil. (Samples of the new fluids were 

 exhibited.) 



Professor Abbe has furnished Mr. Zeiss with a new formula for 

 homogeneous |.ths having a numerical aperture of 1 • 40 and adjusted 

 for the new fluids. 



Mr. John May all, jun., said that at the December meeting he 

 exhibited a stage constructed by Mr. Tolles, which he then stated to 

 be the thinnest that had been made (see p. 115). It appeared, how- 

 ever, that Messrs. Watson had, quite independently of Mr. Tolles, 

 made a stage of a similar kind, which he now exhibited. The stage 

 was thinner than that of Mr. Tolles, so that it appeared to be really 

 the thinnest mechanical stage yet made (see p. 300). 



Mr. Crisp exhibited two Microscopes by Messrs. Swift and Son, 

 the first being their " Challenge " model, fitted with Radial Traversing 



