52 ON THE CONSTRUCTION AND USE OF 



parent, that although a minute object placed in one focus was perfectly reproduced 

 at the other, seven times as large, yet a large one was not equally well defined in 

 all its parts. 



I determined then to produce the enlarged image by passing a solar-beam 1 5 inch 

 in diameter through the original lunar negative placed in the focus nearest to the 

 mirror and allowing it to fall on a portion of the concave mirror, 1 1 inch in 

 diameter, at one side of the vertex. Being reflected, it returns past the negative 1 , 

 and goes to form the magnified image at the other focus of the ellipse. 



In Fig. 46, a is the heliostat on a stone shelf outside ; b a silvered glass mirror, 

 to direct the parallel rays through c, the negative ; d is the elliptical mirror ; < an 

 aperture to be partly closed by diaphragms; /a rackwork movement carried by the 

 tripod (f ; the curtain h h' shuts out stray light from the interior of the observatory. 

 The aperture i is also diaphragmed, but is shown open to indicate the position of 

 the heliostat, the shelf of which joins the outside of the building at 1. The dotted 

 line points out the course of the light, which coming from the sun falls on the 

 heliostat mirror a, then on b, through c to d, and thence returning through e to the 

 sensitive plate in the plate holder k. 



The distance of this last can be made to vary, being either two feet or twenty- 

 eight feet from d. In the latter case a magnifying power of about 25 results, the 

 moon being made three feet in diameter. The sensitive plate is carried by a frame, 

 which screws to the side wall of the building, and can be easily changed in position. 

 The focussing is accomplished by the rack /. Where so small a part (1| inch) of 

 the surface of the mirror is used, a rigid adherence then to the true foci of this 

 ellipse is not demanded, the mirror seeming to perform equally well whether magni- 

 fying 7 or 25 times. Theoretically it woidd seem to be limited to the former 

 power. 



If instead of placing a lunar photograph, which in the nature of the case is never 

 absolutely sharp, at c, some natural object, as for instance a section of bone, is attached 

 to the frame moved by f, then under a power of 25 times it is as we'll defined as in 

 any microscope, while at the same time the amount of its surface seen at once is 

 much larger than in such instruments, and the field is flat. If the intention were, 

 however, to make microscopic photographs, a mirror of much shorter focal length 

 would be desirable, one approaching more to those of Amici's microscopes. 



By the aid of a concave mirror used thus obliquely, or cxcentrically, all the diffi- 

 culties in the way of enlarging disappear, and pictures of the greatest size can be 

 produced in perfection. I should long ago have made lunar photographs of more 

 than 3 feet in diameter, except for the difficulties of manipulating such large sur- 

 faces. 



In order to secure a constant beam of sunlight a heliostat is placed outside the 

 observatory, at its southeast corner/, Fig. 38. This beam, which can be sent for 

 an entire day in the direction of the earth's axis, is intercepted as shown at f>, Fig. 

 46, and thus if needed an exposure of many hours could be given. The interior of 

 the observatory and photographic room being only illuminated by faint yellow rays, 

 no camera box is required to cut off stray light. The eye is by these means kept 

 in a most sensitive condition, and the focussing can be effected with the critical 



