314 The Microscope. 



The possibility of obtaining witli these new objectives very- 

 long foci, and of anj' length desired, is very easily explained. 

 With convergent lenses the farther the object seen the nearer 

 to the principal focus is the image on the other side of the lens, 

 so that if it is wished to receive it on a screen, or to examine it 

 with an optical apparatus, it is necessary to approach the 

 nearer to the principal focus. Reciprocally, when very near the 

 lens, it is only possible to see the images of very distant objects ; 

 and, on the other hand, when receding from it, only those of 

 objects very near. Similarly, with this new system of objec- 

 tives, if the second lens is brought near the first, only very dis- 

 tant objects can be seen, and accordingly the focal length of the 

 whole system will be augmented, while by separating the lenses 

 the focal length will be diminished, and only nearer objects can 

 be seen. 1 have made one of these objectives in which the two 

 lenses can be approached or separated at will, so as to vary at 

 pleasure the length of the foci, and to see with the microscoj^e 

 objects more or less distant. In practice, however, I think that 

 it is better to use objectives with fixed focus. 



The idea of erecting microscopic images by means of the ob- 

 jective is not new. The first of these new objectives was con- 

 structed by me eleven years ago, and was shown then to many 

 persons, amongst whom was M. Verick, who undertook to make 

 similar ones. He did not do so, but his successor has been en- 

 gaged under my direction in this new work. Any maker will 

 be able easilv to do the same after some trials. 



WAYS • 

 AND MEANS 



Another stage ledge. — To a plain stage clips are perhaps a 

 necessary evil. They may " scalp off the cover," rub off labels, 

 hold the slide down too tight, or so loosely that it gets away 

 from them, according to the thickness of slide used. When 

 using the microscope horizontall}'' one can very well do without 

 the clips, but in the convenient inclined position something is 

 absolutely necessary to hold the slide in place. 



