10 Prof. Petzval on the Camera Obscura. 



as was in general use in the early period of daguerreotyping, the 

 diaphragm is best placed at a distance of 3 inches before the 

 lens, its aperture being 1 inch. The image thus obtained, 

 although tolerably good, will uot be of uniform sharpness ; in 

 the centre it will perhaps bear magnifying three times, whilst at 

 the edges it will barely admit of examination with the naked eye. 

 In point of sharpness, therefore, this picture is at least three 

 times inferior to the one of the camera already used as a term of 

 comparison. "With respect to illumination, the superiority of the 

 ordinary modern camera is still greater ; for since the degrees of 

 illumination are directly proportional to the squares of the aper- 

 tures, and inversely proportional to the squares of the focal 

 lengths, the ratio in question is 



12 X 11^ : 3^ X 16- = 131 : 2304. or 1 : 19 nearly. 



It must be noted, however, that the modern camera has four 

 more reflecting surfaces than the old one, by which means 

 almost one-fifth of the light is lost, and the above ratio dimi- 

 nished to about 1 : 10. 



The substitution of au achromatic in place of an unachromatic 

 object-glass is, beyond comparison, the most important step in 

 the improvement of the camera ; for not only have the proper- 

 ties of sharpness and illumination been thereby increased — the 

 former iji the ratio of 1 : 7, and the latter even in the ratio of 

 1 : 40^ — but the serious defect of separated optical and chemical 

 foci has been remedied. Besides this, the image has become 

 nearly plane — a result which, it is true, might also have been ob- 

 tained in the case of an unachromatic lens by means of the same 

 method of blinding. Lastlj^, the field has become almost uni- 

 formly lighted ; the not very broad zone of diminishing inten- 

 sity of light which still exists is due to the blinding. As dia- 

 phragms often produce this defect, it will be well to examine 

 their action more closely. 



Around the centre of the lens, and with a radius of 1 inch, 

 conceive a circle to be described : its circumference will be at the 

 distance of half an inch from that of the lens. The diaphragm, 

 at the distance of 3 inches, having an aperture of 1 inch, all 

 cylinders of rays passing through the same will be entirely re- 

 ceived by the lens, provided their axes are within or upon the 

 circumference of the above circle, and the corresponding images 

 will possess the maximum intensity of light. The rays of every 

 cylinder whose axis meets the lens in the circumference of the 

 circle are inclined to the axis of the instrument at an angle whose 

 tangent equals i, and whose magnitude is therefore 18°; conse- 

 quently everywhere within a field of 36° the image possesses full 

 intensity of Ught. Again, only half the rays of those cyhuders 



