46 PRACTICAL MICROSCOPY. 



Illustrations,' that " microscopes are now placed completely 

 on a level with telescopes and, like them, must remain 

 stationary in their construction." 



Improvements, however, continued to be effected ; Mr. 

 Thomas Ross, upon increasing the air angle, discovered 

 that different thicknesses of covering glass disturbed the 

 corrections for spherical and chromatic aberrations no 

 matter how carefully made, and in 1837 he presented a 

 paper to the Society of Arts upon the subject. In this 

 paper he stated having made an improved combination, the 

 focal length being one-eighth of an inch, with an air angle 

 of 60. After this he announced obtaining an air angle 

 of 135, and falling into a similar error of dogmatism as 

 Goring, Wollaston, and Biot, stated that "135 is the largest 

 angular pencil that can be passed through a microscope 

 object-glass." 



In 1851, Chas. A. Spencer, of Canastota, N.Y., pro- 

 duced objectives of 146 air angle, and in 1857 he con- 

 structed a one-twelfth with an angle of 178. Since this, 

 Mr. Tolles, of Boston, has made lenses claiming to be 

 infinitely near 180, and this angle in air has been 

 approached by several of the best English makers. 



But these are not the whole of the improvements which 

 have been effected ; air lenses or dry objectives have been 

 supplemented by water-immersion powers, and finally we 

 have the homogeneous-immersion system, in which the trans- 

 mitted ray pursues a rectilinear course from the under side 

 of the object slide until it leaves the posterior surface of the 

 front lens. These immersion lenses will be fully described 

 later on ; all we wish to state here in reference to them may 

 be said in the words of Prof. Abbe : " A wide angle immer- 

 sion glass may therefore utilise rays from an object in a 

 denser medium which are entirely lost for the image which 

 in fact do not exist when the same object is in air, or is 



