CORRESPONDENCE , 



155 



from tlie position of the object, that each ray of light passing from 

 that point through the surface of the hemisphere will be transmitted in 

 straight lines in a radial direction without undergoing any refraction ; 

 the consequence of this is that the full and undiminished aperture of 

 the object-glass is made to bear upon the object." 



In this there is no need of abstruse calculations, or diagrams from 

 high mathematical authorities ; for such if they do not correspond with 

 actual results only tend to confusion, for by this diagram it can be 

 seen at a glance that any aperture existing in the back combination is 

 directly transmitted to the object in balsam without loss from refrac- 

 tion. If the radius of the immersion front is lengthened or falls 

 beyond the object, then the angle of the back combination being re- 

 fracted by a flatter surface, will become diminished. 



As it may be argued that this only suggests a principle and is not 

 a practicable object-glass, inasmuch as the front lens even though 

 set in place was not attached to the cell as part of the system, I hav^e 

 therefore just finished a cell-adapter with a front lens of this descrip- 

 tion, applied to a combined immersion and dry ith of fine quality, and 

 the result confirms the high opinion that I formed of it when the idea 

 first occiu'red to me.* When used as an ordinary water immersion 

 object-glass, I have yet seen nothing that equals it on tests in balsam. 

 It also acts perfectly as an immersion on objects mounted dry ; and, 



* '' When an object is seen under these circumstances, it at once shows the 

 great increase of distinctness that is to be obtained in the structure of the more 

 difficult diatomaceous tests when they are thus viewed in Canada balsam, with 

 the full aperture of the object-glass: markings which in the neighbouring c/r// 

 objects of the same character are scarcely discernible, are sharply and distinctly 

 visible under the hemisphere with the same illuminatiou." — ' Quarterly Journal 

 of Microscopical Science,' July 1855, No. xii., p. 304. 



