The Microscope. 313 



see with the microscope objects placed at#the other end of the 

 work-room, or even objects more distant still, such as houses 

 and monuments, at a distance from the window. However, as 

 we lose in magnification and light what we gain in length of 

 focus, it is of advantage to limit this as much as possible. 



These new objectives possess the further advantage of con- 

 siderable penetrating power, i. e., it is possible to var^^ the focus 

 without losing the object. The one mentioned above has, for 

 instance, a penetration of two to three millimetres. It is possi- 

 ble to get more, but it is necessary to limit it, for it would be at 

 the expense of the defining power, i. e., at the expense of the 

 clearness of the images. 



The field of view is sufficiently large ; that of the objective 

 already taken as an example is from eight to ten millimetres in 

 diameter. With it microscopic images are obtained perfectly 

 plane. The field is of course enlarged, as the magnification is 

 reduced. The device by which I have obtained the two princi- 

 pal properties characteristic of this new system of objectives, 

 viz., the erection of the images and the indefinite length of the 

 foci, is as follows : — 



The different lenses composing the objectives really form two 

 distinct optical systems, each acting as a single convergent 

 lens. One, called, for convenience of description, the first lens, 

 occupies the lower part of the objective next the object to be 

 examined, while the other, called the second lens, occupies the 

 upper part in connection with the microscope tube, 



' Matters are so disposed that the first lens gives behind it and 

 in front of the second an inverted image of the object, and the 

 second then gives behind it an inverted image of the first. It 

 follows from this that this second image, inverted in relation to 

 the first, is really erect in respect to the object. As this is the 

 image examined by the eye-piece which does not invert, it 

 accordingly remains erect with respect to the object. In other 

 words, the aim of the first lens is to give an inverted image of 

 the object ; while the second acts as an ordinary objective, and 

 with the eye-piece constitutes a compound microscope, so that 

 we examine with this microscope not the object itself, but an 

 inverted image of it, produced by a lens placed in front of the 

 objective, between it and the object. The microscope, as it in- 

 verts anew this inverted image, gives a final image whi'jh is 

 erect with respect to the object examined. 



