752 APPENDIX. 



It is not difficult to see that Amici's method of connecting 

 the objective with the cover-glass by a film of water must 

 necessarily very much dimmish the reflection which usually 

 takes place in the incidence of oblique light when the 

 ordinary or dry objective is employed. The limiting angle 

 of refraction of water being about 48 degrees, it follows 

 that whatever is the degree of obliquity in the incident 

 light falling on the object, the immersion lens never has 

 to deal with rays of greater obliquity than 48 degrees. 

 To this, as well as the greater number of parallel rays 

 brought to a focus, and the increased angle of aperture, 

 is due the greater clearness and precision of the image 

 obtained. 



" The advantages," writes Mr. J. Mayall, 1 " mainly 

 claimed for the immersion objectives are : greater work- 

 ing distance between the object and the objective, increase 

 of light, superior definition and clearness in the optical 

 image, which image is obtained by much simpler illumi- 

 nating apparatus, and with less manipulative skill than 

 that considered indispensable in using high-power dry 

 objectives." 



We are pleased to add that Messrs. Powell and Lealand 

 are turning their attention to the construction of objectives 

 on the immersion system. This cannot however be effected 

 1 y making the front lens moveable, so that the objective 

 may be used either wet or dry. 



(1) The Monthly Microscopical Journal, 1869, p. 92. 



