OP THE MICKOSCOPE. 151 



a permanent part of the particular objective employed, and be 

 specially adapted thereto. In our own experience, the best 

 results have been obtained by the latter arrangement. 



Instead of the silver reflector, Mr. E. Beck uses a thin glass 

 plate (an ordinary cover-glass), which is inserted into an adapter 

 which fits between the nose-piece and the objective. The thin 

 glass is supported by a small pin with a milled head, by which 

 it may be turned so as to present its surface at the best angle 

 for reflecting downward the light admitted through a suitable 

 aperture. 



All forms of the vertical illuminator give their best results 

 when used with immersion objectives, as has been very fully 

 shown by Mr. Geo. W. Morehouse, of Way land, N. Y., who 

 uses with success objectives as high as the one-tenth. The 

 markings on the most difficult tests even the 19th band of 

 Nobert's plate are shown clearly and well by this arrange- 

 ment. 



In using the vertical illuminator, success will depend greatly 

 upon the management of the illumination. The size of the 

 aperture which admits the light to the reflector should be 

 carefully regulated and diaphragm ed down, if necessary, and 

 the rays should be rendered parallel by means of the bulls-eye 

 condenser. 



Tolles' Vertical Illuminator. So far as we have been 

 able to find, Prof. H. L. Smith was the first to illuminate the 

 object by light passed down through the objective, and the 

 different forms which have been introduced, such as Beck's, 

 Powell & Lealand's, etc., are mere modifications of his original 

 plan. To Mr. Tolles, however, is due the invention of a modi- 

 fication which exhibits considerable originality. He inserts, 

 above the front lens, a small prism, which is so constructed 

 that the light passes in freely, and is then totally reflected 

 downwards. In this way the rays pass down through only one 

 lens of the system, instead of through all of them. Mr. W. A. 

 Eogers has used this illuminator with very satisfactory results 

 in the examination of fine rulings on metal, incident to his 

 investigation of the comparative value of various standard 

 linear measures. 



