240 MEMORANDA. 



The refractive effect of balsam causes but a small compara- 

 tive difference in favour of extreme degrees : for example, 

 referring to my former experiments, two object-glasses, one of 

 146° and the other of 105" of aperture, will be reduced over 

 an object in balsam to 75° and 68^, being a difference of 

 forty-one degrees in air, but only seven degrees in balsam. In 

 discussing this subject, I have omitted to mention that balsam- 

 mounting not only has the effect of another optical combina- 

 tion by the refractive medium reducing the aperture, but that 

 the same I'efraction also slightly diminishes the magnifying 

 power of the object-glass. This may be easily proved by 

 measuring the length of an object both before and after fluid 

 balsam has been allowed to run under the thin glass cover, 

 using a l-12th objective. — F. H. Wenham. 



Application of Collodion to the Production of Stage and Eye-piece 

 Micrometers for the Microscope. — A cheap stage-micrometer may 

 be made by taking a cast in collodion from a piece of ruled 

 glass, and mounting the cast thus taken as a microscopic 

 slide. The specimen sent with this is one of many casts 

 taken in this way from a piece of glass on which lines had 

 been ruled at the distance of l-400ths of an inch. Every 

 irregularity in the original is accurately copied on the collo- 

 dion ; and by this planone correctly-ruled micrometer on glass 

 may be made to furnish any require dnumber of exact copies. 



An eye-piece micrometer may be made in a very simple 

 manner, by employing the photographic camera to reduce a 

 coarsely-graduated original to any degree of ininuteness which 

 may be desired. The specimens sent with this are copies of 

 a scale of inches and tenths, reduced in this way either to 

 tenths and hundredths or to twentieths and two-hundredth s ; 

 and also of a diagonal scale reduced so as to furnish for the 

 eye-piece of the microscope a microineter of which the divi- 

 sions are hundredths and thousandths of an inch. The 

 originals are of glass, covered with black paper or black 

 varnish, on which lines were drawn with a knife-point. 

 These micrometers are not perhaps so sharp as those ruled 

 by machines, but they may be made at a much smaller cost, 

 of any required pattern and size, and by those who have no 

 machines within their reach. — W. Hodgson, Old Brathay. 



Note on Pinnuiaria.^ — At the first page of this volume of the 

 ' Quarterly Journal of Microscopical Science,' Dr. Gregory, 

 noticing a Pinniilaria, for which he had adopted Professor 

 Smith's name of Pinniilaria latestriata, proceeds thus: — '"I 

 could find no figure of this species in any work to which I 

 had access, neither in Ehrenberg's Atlas in 1838, in Kutzing, 



