82 THE MICROSCOPE. 



balancing certain uncorrected aberrations. Soem- 

 mering, writing of one of Arnici's microscopes, ob- 

 serves : " The magnifying power, admirable precision, 

 and clearness with which the object is seen, seems to 

 me quite astonishing." It is not difficult to perceive 

 that, this optician's method of connecting the objective 

 with the cover-glass of the object by means of a drop 

 of water should diminish the reflection which takes 

 place in the incidence of oblique light when the dry 

 objective is used. The limiting angle of refraction 

 of water being nearly 48, it follows that whatever 

 the degree of obliquity in the incident light falling on 

 the object, the immersion lens can never have to deal 

 with rays of greater obliquity than 48. To this 

 circumstance, as well as to the greater number of 

 parallel rays brought to a focus, and to the increased 

 angle of aperture is due the greater clearness and pre- 

 cision and sharpness of the image. The film of water 

 not only furnishes increased angle of aperture, but it 

 also collects the straying away peripheral rays of light, 

 and sends them on to the eye-piece, to assist in render- 

 ing the image more perfect ; becomes, indeed, an in- 

 tegral part of the optical system, and very materially 

 aids in the removal of residuary secondary aberrations. 

 The water-immersion system was warmly advocated 

 and fully developed by continental makers Hartnack, 

 Merz, Nachet, and others long before English opticians 

 could be persuaded to acknowledge its advantages. 

 Messrs. Powell and Lealand were the first opticians 

 who made a ^-inch and a |-th objective, which, by a 

 mere change of the front lens, could be used either 

 as a wet or dry lens. 



The immersion principle has recently been still 

 further developed. The substitution of oil for water 

 was first proposed by Amici, in 1844, who abandoned it 

 as it seemed unmanageable and without correspond- 

 ing advantages as compared with water, an opinion 

 shared by Oberhauser and Wenham. At this time, 

 however, it was supposed that the chief gain of the 

 immersion consisted in a diminished loss by reflection 

 at the front lens and an increase of working distance ; 



