332 SCIENTIFIC APPARATUS. 



correcting residual imperfections in the object-glasses, and in 

 giving a flat and even field. The magnifying power of the entire 

 instrument depends in part on that of the eye-lens of the eye- 

 piece; but practically the limit of satisfactory results is soon 

 reached. It is convenient to have two or three eye-pieces of 

 increasing power, but beyond a certain point any increased 

 advantage due to the higher magnifying power is entirely out- 

 weighed by loss of light and good definition ; and it is far better 

 never to use eye-pieces of very high power, but to employ those 

 of more moderate power in combination with object-glasses of 

 shorter focal length and greater aperture. This, however, neces- 

 sarily very much increases the cost of the instrument. 



In order to overcome the difficulties arising from the inverted 

 image in dissecting under a compound microscope, erecting glasses 

 are sometimes used, which give the object in its true position by 

 inverting the image a second time. 



BINOCULAR MICROSCOPES. 



What is desirable is an instrument with which both eyes can 

 be easily used without any strain, having the field of view down 

 both tubes complete on all sides, with the definition unimpaired, 

 and -so constructed that at pleasure the whole of the light can be 

 sent directly up one tube, and the instrument used as an ordinary 

 monocular microscope. In practice it is, however, very difficult, 

 if not impossible, to combine all these advantages, and one or 

 other must generally be sacrificed, according to the taste of the 

 observer or the work with which he is engaged. After passing 

 through the object-glass the light is divided into two portions by 

 means of a prism which reflects one half of the light up an 

 oblique tube, whilst the other passes outside the prism directly 

 up the other tube, or else it is divided by two prisms into two 

 portions, which are deflected up two separate tubes both 

 inclined to the direct line. With this latter arrangement certain 



