On Tones' Interior Illuminator. Bij William A. Bogers. 751 



I will close this 



Fig. 73. 



I find the prism useful in supplementing the light from the 

 mirror below, when an intense illumination is desired with trans- 

 parent objects. This method of illumination seems to be rather 

 better adapted to high than to low powers. I have a ^ with 

 which the most perfect illumination of graduated metal surfaces 

 can be obtained by simply turning the face of the prism towards 

 a window. This method seems well adapted also to the resolu- 

 tion of bands of fine lines. If the lines are ruled on cover- 

 glass, and are covered with a thin coating of either silver, gold, or 

 platinum by the method of Professor Wright, of Yale College, the 

 resolution will be efiected about as well by looking at the lines 

 through the coating as by viewing them by reflection. 



The method of illumination here described has an especial 

 interest in connection with immersion objectives. 

 article with a communication with which Mr. 

 Tolles has kindly furnished me, together with 

 the sketches shown in Figs. 73 and 74. 



" With immersion objectives the illumi- 

 nator-front has still more efiective and extended 

 application : — 



First, Because more of the front lens can 

 be brought into use for the purpose of illumi- 

 nation than with dry objectives. 



Second, Because any possible glare arising 

 from the marginal zone of total reflection in 

 the dry objective, has no existence when the 

 front has water contact with the covering 

 glass. This is strictly true in the case of the ';/ 



prism, while it might not be true in the case of r 



a transparent disk of glass, placed as a reflector 

 at the back of the entire objective system, and covering its entire 

 aperture. Eeflection from a disk might easily reach an outside zone 

 of total reflection even with a water-immersion front, and give back 

 stray rays which would cloud the view, but the prism would neces- 

 sarily stop all rays not contributing to the formation of the image, 

 even without the interposition of diaphragms. In the case of bands 

 of lines, as in Nobcrt's plates, there would be for the most part 

 exemption from glare, and the whole interior aperture of the 

 objective would bo brought into use, except that portion which is 

 stopped by the prism. • The angle of this interior aperture would 

 be bounded i7i a homogeneous immersion medium by the extreme 

 rays utilized by the objective. 



Fig. 74 represents the front duplex system of the immersion 

 ■J-inch objective made for Mr. Crisp in 1873. Jt is one of the very 

 first made to demonstrate the practicability and the utility of the 

 outside — ' cxtru-limital ' — immersion aperture. It has an cxcep- 



