630 SUMMARY OF CUEEENT EESEAECHES EELATING TO 



liminary operation is recommended whenever it is desired to compare 

 this instrument with other Microscopes. 



" The objectives are enclosed in a little box, inserting them all 

 together, as they are in fig. 6". If they are inserted in a different order, 

 there is a risk of breaking the glass of one against the other. 



"Florence, 17 Dec. 1841." 



Knipe, O. — The Projection Microscope. 



[An elementary description of method of use.] 



Micr. Bull, Febr. and April, 1900, pp. 1 and i. 



Disney, A. N. — Modern Microscopes. 



[The author reviews the most notable instruments which have been noticed in 

 the Journal of the R.M.S. during last three years.] 



Nature, lxii. (1900) pp. 154-6 (2 figs.). 



(2) Eye-pieces and Objectives. 



Eye-piece Diaphragms.* — M. Malassez has communicated to the 

 Societe de Biologie several inventions of his own for obtaining certain 

 cheap accessory apparatus. One of these is an Ocular Diaphragm with 

 movable index. In the eye-piece at the position of the ordinary dia- 

 phragm he inserts a disc of blackened cork with a kind of watch-hand 

 pointer ; one extremity (ring-shaped) is pivoted by a pin-head near the 

 periphery of the disc, the other extremity (finely pointed) projects into 

 the field. A rotation of the eye-pieces exerts sufficient friction to make 

 the hand appear or disappear at will. Thus the position of any point in 

 the Microscope image can be indicated. 



M. Malassez also describes several simple contrivances for procuring 

 effective home-made micrometer eye-pieces ; also a new form of lens- 

 carrier. 



(3X Illuminating: and other Apparatus. 



Abbe's Spectrometer. — This apparatus, originally designed by Prof. 

 Abbe as far back as 1874, has undergone various improvements, and is 

 now made by the Zeiss firm in the form shown in fig. 156. The 

 principle is that known as " autocollimation," whereby the incident ray, 

 after suffering normal reflection at the back face of the prism, issues in 

 the same direction as it entered (fig. 158). In the adjustment of the 

 ocular head (fig. 157) the width of the slit S is regulated from the 

 under side by a screw. The course of the light is from the left through 

 the illuminating prism P. The lower half of the field of view is free 

 for the observation of the spectrum. The adjustment point (the inter- 

 section of two threads crossing at a sharp angle) is applicable both for 

 dark lines on a bright ground and for light lines on a dark one. The 

 focusing of the telescope is by rack-and-pinion : and its normal posi- 

 tion with regard to the rotation axis of the graduated circle is ob- 

 obtained by the adjustment screw. 



For determining the refractive index, Abbe's method has all the 

 advantages of Fraunhofer's without its disadvantages. For the ray-path 

 (fig. 158) is exactly identical with* the path of a ray in the minimum 

 of deviation through a prism of twice the refractive angle. But the 

 mode of measurement is essentially simpler. It is only necessary to 

 rotate the prism just so far round the vertical axis of the spectrometer 



* C.K. Soe. de Biol., lii. (1900) pp. 629-33, 724-7. 



