562 Notes. 



at a considerable distance from the lenses, and even if the focal length of 

 the complete instrument were xrftns incli there is ample room for manipulat- 

 ing the object. The modern compound Microscopes consist of two positive 

 lenses separated by a large interval, and the two equivalent planes are 

 outside everything ; the object to be examined and the observer's eyes are 

 inside. 



Mr. Beck then urged the importance of examining instruments as a 

 complete whole with a view to seeing if, by re-arrangement of parts, new 

 positions could not be found in which the equivalent planes might be placed 

 in order to confer fresh capabilities, and concluded by treating the case of 

 the human eye and spectacles, deducing several curious facts as to the size 

 of the picture received upon the eye, firstly, in cases of high myopia cor- 

 rected with spectacles, and secondly, in cases of cataract with the crystalline 

 lens replaced by powerful convex spectacles. 



On Certain Methods of Lens Measurement and Testing together, 

 with some Recommendations as to Nomenclature and Description. 



By T. H. Blahesley. 



The instrument used is in essence a collimator, for it consists of a scale 

 of a few divisions at the principal focus of an achromatic lens. It is fitted 

 to the stage of a Microscope, being placed through the hole in the stage, 

 upon which it rests by means of a collar near the lens. The collimator lens 

 itself is turned towards the Microscope, and the lens or lens systems, the 

 measurements of which are to be determined, are placed between this colli- 

 mating lens and the Microscope objective. The method may be employed 

 for determining the focal lengths of lenses, the distance between the princi- 

 pal focus from surfaces of a lens, the ratio of the radii of the surfaces of a 

 single lens, the distance between the second principal focus of one lens and 

 the first principal focus of the second lens in a combination of two lenses, 

 the curvature of lens surfaces, and it has also been applied to measure the 

 index of refraction of a liquid. With regard to determining the focal length 

 of lens systems, the method was found applicable to an entire Microscope, 

 the tube of which was not long. 



*Aberrations. 

 By S. D. Chalmers, M.A. 



The author discusses practical means for measuring by observational 

 methods the aberration of lenses, more particularly photographic lenses, on 

 the system employed by Hartmann for telescope objectives. It is pointed 

 out that it has not been possible, as yet, to apply the method to Microscope 

 objectives with sufficient accuracy. 



*Measurkmbnt of Befractive Index. 

 By S. D. Chalmers, M.A. 



The paper describes a new and accurate method of determining the re- 

 fractive index of glass in the form of a lens. 



The lens to be tested is inserted in a liquid whose refractive index 

 can be measured, and the refractive index is determined from the formula 

 n z - n (Bj - B ;) ) = the power of the lens (when immersed in the liquid). 

 Where n 2 is the refractive index of the lens, n of the liquid, B, and E, the 

 curvatures of tli e lens; the latter need only be known approximately. To 



