IMMERSION OBJECTIVES. 121 



The value of wide pencils was first put into practical 

 form by Hartnack, who made in the ordinary course of busi- 

 ness water-immersion objectives utilising pencils of light 

 approaching to 170 in air; a drop of water was placed be- 

 tween the objective and the object, and thus by passing 

 through a denser medium, rays of light entered the object- 

 glass which could not possibly enter from air, giving at the 

 same time a considerable working distance for the higher 

 powers. Some years afterwards Mr. J. W. Stephenson, 

 whose name has been already associated with the erecting 

 binocular, conceived the idea of substituting for water, in the 

 immersion objective, a fluid having the same refractive and 

 dispersive power as crown glass ; fourfold systems upon 

 this plan were calculated by Professor Abbe and made by 

 Carl Zeiss, the optician of Jena, of ^ and -|- equivalent 

 foci. These objectives had a balsam angle of 113: 

 greater than the maximum of 180 in air, in the ratio 

 of 5 to 4. 



Messrs. Powell and Lealand have recently constructed a 

 homogeneous - immersion objective of T Vmch equivalent 

 focus, with a numerical aperture of I "43 or 140 balsam 

 angle, with two extra fronts, one of which gives an aperture 

 of I * 28 or 1 1 5 balsam angle, while the other provides an 

 aperture of 1*0, or 82 in balsam. With an aperture of 

 ,1*43 the working distance is 0*007 inch; the aperture of 

 I -28 gives a focal distance of 0*016 inch ; while with the 

 numerical aperture of 1*0 the working distance is 0*024 

 inch. 



It was the existence of these immersion objectives and 

 their various angular values which led Professor Abbe to 

 investigate the general principles of microscopic vision. 

 He tells us that "the very first step of every understanding 

 of the microscope is to abandon the gratuitous assumption 

 of our ancestors, that microscopical vision is an imitation of 



