130 



SUMMABY OF CURKENT RESEAKCHES RELATING TO 



transfer the instrument with focus adjustment to the hands of some 

 one unused to a lens, that even that provision is necessary practically, 

 for anything not exceeding 150 or 200 diameters." 



Mr. Hippisley also says that he makes "Coddington" lenses by 

 melting pear-shaped pieces of glass until the ends in advancing 

 towards a spherical form have approached to the right distance, 

 which is ascertained by repeated trials. As the two ends cannot, 

 except by chance, be exactly of the same curvature, one end has 

 to be selected and marked, as that to which the eye is to be 

 applied. 



Griffith's Substage Diaphragm.— Mr. E. H. Griffith's substage 

 diaphragm is intended as a substitute for the cheaper kind. The 



principal claims for it are that it 

 Fig. 13. will do the work well that is re- 



quired of much more expensive 

 ones, and as it is placed in the 

 centre of the substage fitting, and 

 so constructed that it may be 

 turned in any direction, many 

 effects may be secured by simply 

 moving the slide. Being central it 

 is not so much in the way as some 

 other forms. 



It is simply a perforated metal 

 button fitting the Society screw 

 of the substage. Through the 

 head is a groove, cut with a 

 milling-machine, which is pro- 

 vided with a diaphragm slide 

 which has different sized and shaped apertures which can be placed 

 exactly central by means of stops, or out of centre if desired. The 

 slit can be made to be perpendicular, diagonal, or longitudinal to the 

 slide, as desired, by turning the button. 



Sorby's Direct Illuminator. — ^In some recent discussions on the 

 microscopical structure of metals. Dr. H. C. Sorby has recalled 

 attention to the illuminator devised by him many years ago for the 



examination of minerals. It consists 

 (fig. 14) of the " Parabolic Eeflector," in 

 the centre of which, in a semi-cylin- 

 drical tube, open in front, is placed a 

 small plane reflector which covers half 

 of the objective, and throws the light 

 directly down upon the object, and back 

 through the other half. This allows of 

 two kinds of illumination, oblique and direct, to be readily used, as 

 the plane reflector is attached to an arm so that it can be swung out 

 of the way when not required, as shown by the dotted lines in the fig. 

 Dr. Sorby writes : — " I may say that for the study of polished and 

 etched sections of iron and steel, it is almost indispensable. In 



Fig. 14. 



