43° HISTORY OF LENSES AND MICROSCOPES [Ch. XII 



ocular (fig. 248). The convex lens is placed to give a real image of 

 the object, that is, the object is outside the principal focus of the 

 objective (fig. 248). But before the real image is formed a concave 

 lens (the ocular) is placed in the path of the beam. This makes 

 the rays less convergent and therefore acts as an amphfier to in- 

 crease the size of the real image of the objective. The eye is placed 

 close to the ocular and focuses the real image on the retina. This 

 retinal image is inverted (fig. 5-6) and therefore when pro- 

 jected out into space it seems erect (fig. 248), as with the simple 

 microscope (fig. 182). 



From the testimony of eye-witnesses this form of compound micro- 

 scope was devised by a spectacle maker in Middleburg, Holland, about 

 the year 1590, the name of the inventor being Zacharias Jansen. 

 (See Borellus.) 



Very early the two lenses were put into tubes and made capable 

 of being brought together or separated, depending upon the distance 

 of the object to be examined. The nearer the object the farther apart 

 must be the ocular and objective. There still remains in the ordinary 

 opera glass the original Dutch telescope. If one has an opera glass 

 it is easUy demonstrated that it can be used as a microscope by un- 

 screwing the ocular so that it may be separated a considerable distance 

 from the objective. If now the objective is held within 10 to 20 

 centimeters of an object and the ocular moved back and forth along 

 the axis, the place will be soon found where the image is distinct and 

 it wiU be seen much enlarged. 



The name telescope was given sometime before 16 18, and the desig- 

 nation microscope in 1825 (§ 2a). As every one who used the instru- 

 ment found that it could be used as a microscope or as a telescope 

 it soon came to be called a telescope-microscope, or a microscope- 

 telescope. 



§ 696. The Keplerian compound microscope. — When the Dutch 

 telescope came to the attention of the astronomer and optician, 

 Kepler, he very quickly saw that the same effect could be brought 

 about by using a convex ocular as well as a convex objective, but that 

 the image would be inverted, the objective serving to produce an 

 enlarged real image and the ocular to magnify that image. 



