PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 445 



the included angle is seventy-five degrees, in a twelve-inch globe (that is 

 one of tw^elve seconds focus) .the included angle is over ninety degrees. It 

 has been said that the light, being admitted through a round hole in a 

 plane plate in the centre of the instrument, must be much more intense at 

 the centre of the field than at the margin, and some writers have stated 

 this fault to be one of great magnitude. Practice however does not show 

 the evil to be so great as they represent, if it exists at all. By reference 

 to the cut, it will be seen, that the dotted lines x x, representing a pencil 

 of light of the diameter of the centre opening passing through the ax's of 

 the instrument, and y y representing a pencil of light passing through the 

 same opening obliquely, doubtless the area of the centre one will be the 

 largest, but as it passes through much thicker glass than the rays y y, may 

 not the ultimate effect of each be equalized? I do not pretend to any 

 knowledge of the theory of optics, and must confine myself to facts. In 

 the trial of many of these lenses, of different sizes, I have never found the 

 evil to exist, and all the pictures I have made with the globe lens are re- 

 markable for the even illumination of the field. At the last two meetings 

 of the Photographic Society of Philadelphia, (February, 1863,) the merits 

 of these lenses have been discussed — full credit for correctm ss of repro- 

 duction was accorded to them by all; but the quickness of working was 

 questioned by one gentleman, who stated that in broad sunlight he had 

 exposed an engraving for several minutes and had obtained an under- 

 exposed negative, while all others present who had tried them were unhesi- 

 tating in their assertions that they were remarkably quick workers when 

 the size of aperture was taken into consideration. xVfew days ago I placed 

 in bright sunlight an engraving from the London Art Journal, and copied 

 it on a six and a half by eight and a half plate with the same size instru- 

 ment as was used by the gentleman who questioned its quickness. An 

 exposure of twenty-five seconds gave an impression which flashed up 

 instantly under the developer, assuming great intensity in the. light and 

 showing unmistakable symptoms of over exposure, so that I can see no 

 reason why the same law should not hold good with these lenses as with 

 others, viz: that, with the same area of opening to admit light, the shorter 

 the focus the quicker they will work. For interiors, the short focus and 

 large angle of vision possessed by these instruments render them invalu- 

 able, and as they are provided with a revolving diaphragm plate in the 

 centre (not shown in the cut), various sizes of aperture can be brought into 

 position, just as the stops under the table of some compound microscopes 

 are arranged, and thus the largest amount of light, consistent with sharp- 

 ness, admitted. 



In the English journals, when the accounts of this instrument were first 

 published, it was denounced in no measured terms, as being constructed 

 on erroneous principles, and the assertion has even been made that its 

 very shape must give fearful distortion to marginal lines, but since it has 

 been proved to be no failure, and its success is no longer an experiment, 

 comes the unwilling acknowledgment: "The principle of its construction 

 must insure correct marginal lines," and last of all comes the declaration 

 that it is "old, very old." Everybody had been making them for years, 

 and there is no merit of invention due to the patentees I Granting that 



