438 HISTORY OF LENSES AND MICROSCOPES [Ch. XII 



inventor; but he not only published the formula, he constructed 

 objectives on it." The formula was submitted with the objective 

 in 1874. The homogeneous immersion objectives of Zeiss came out 

 in 1878. 



Many substances have been tried for the homogeneous fluid. 

 Thickened cedar-wood oil has proved most satisfactory. Mr. Tolles 

 used Canada balsam; if one gets out of cedar- wood oil and has Canada 

 balsam of moderate thickness, good results can be obtained by using 

 the balsam as an immersion liquid. 



§ 704. Projection microscope. — The production of real images 

 by means of a naked aperture and by means of a lens were the begin- 

 nings of the magic lantern, the photographic camera, the projection 

 microscope, and the drawing camera. 



As shown elsewhere (Optic Projection, p. 673), the production of 

 real images in dark places by means of an aperture or hole in the wall 

 is a purely natural phenomenon. The systematic utilization of this 

 phenomenon by man had its beginnings in the sixteenth and seven- 

 teenth centuries. The first certain statement of the use of a lens in 

 the aperture to make the picture clear and vivid occurs in the work 

 of Daniel Barbaro on perspective (§ 705a). 



From this time on a lens is always used for projection. At first 

 the images were smaller than the object, as naturally only the 

 brightly Ughted objects in the exterior world were projected, but as 

 artificial and natural Hght were used to illuminate smaller and smaller 

 objects, many of which were transparent, and the projection lenses 

 were made of shorter focus, the images became larger than the object. 

 Finally (1665), when the apparatus became small, and only the object 

 and lens and light were enclosed and the image was on a screen out- 

 side, the magnifying action seemed Uke that of a microscope, and 

 Milliet de Chales, in speaking of the magic lantern of Walgensten, 

 says (Vol. II, p. 667): " In this machine you have a kind of micro- 

 scope," and Zahn, p. 255, in discussing the magic lantern, says: " It is 

 a kind of a microscope." Both authors point out the great advantage 

 this kind of a microscope has over the ordinary one in that many per- 

 sons can see the image at the same time. Kepler (1611) showed that 

 the Dutch telescope-microscope could be used for projecting images 



