444 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 58. 



A disk at the top of tte eye-piece, with projecting milled edge, 

 carries different eye-lenses which by rotation are brought successively 

 into the optic axis. An aperture in the cap 

 shows by a letter which lens is in place. 



The upper tube to which the eye-glass 

 disk is attached can be drawn out as shown 

 in the figure, and the lower one, in which the 

 field-lens is set, can be similarly drawn out. 



Eings marked B and C show the proper 

 position for each power, and when entirely 

 closed, the eye-piece is of the proper 

 length for a D eye-piece. 



It was intended by the inventor to have 

 a slit with stops for regulating the length of 

 the eye-piece, and that a revolving diaphragm- 

 disk should also be included, but these have 

 not yet been added. 



As to the utility of the eye-piece, it may 

 be pointed out that whilst it would be very 

 convenient to be able to obtain different 

 eye-piece powers by simply rotating a disk, 

 yet most of the advantage is lost by the neces- 

 sity of withdrawing the eye-piece from the 

 tube to alter its length — a process which 

 would occupy as long a time as would be 

 required to insert a different eye-piece. 

 Moreover, it is optically impracticable to make use of the same 

 field-lens for B, C, and D eye-pieces. 



Francotte's Camera Lucida.* — P. Francotte thinks that Beale's 

 camera lucida has a capital defect; the image is formed on the 

 reflector too close to the eye-piece. The consequence is that the 

 whole field is not visible at one time to the eye ; whilst, for instance, 

 the centre can be seen, the periphery is invisible ; and in order to see 

 all parts of the field, it is necessary to move the eye. Besides this, 

 the short space left free between the eye-piece and the glass is very 

 inconvenient. 



To obviate this he replaces the eye-piece by a single lens, giving 

 an image which is reflected by an inclined glass plate or a mirror. 

 The inclination of the reflecting surface may vary between 40° and 60°, 

 according to the point of the table upon which the image is to be 

 projected. The image is erect, and the whole field is included. 



The apparatus can be easily and very cheaply constructed. An 

 ordinary lens (3 to 6 times) in a tube of cardboard is used as the eye- 

 piece. The tube is cut obliquely, so that, on the elliptical section, 

 a thin plate of glass or a mirror may be applied. On the upper 

 surface an opening is made exactly over the place where the image is 

 reflected. 



By adopting the same principle and replacing the large prism of 



* Bull. Soc. Belg. Micr., x. (1884) pp. 77-9. 



