256 HOW TO WORK 



ingenuity, recommended a biconvex lens of low power to be carefully 

 turned down to the proper size, and centered in a setting that can be 

 screwed into the place where the posterior diaphragm or stop is 

 usually placed ; thus to lessen the over correction and to bring the 

 chemical back to the visual focus. He gives the following focal 

 lengths of these correcting lenses for Messrs. Smith, Beck and Beck's 

 i \ inch, a lens of 8 inches focus ; for the frds one of 5 inches focus, 

 which is also applicable to the T \ths. 



Mr. Hislop advises that a dozen of these, of different foci, should 

 be at hand, and the one that is found to answer best in practice 

 selected. Dr. Maddox has one of Messrs. Smith, Beck and Beck's 

 frds, beautifully corrected by them in this manner, and it gives sur- 

 prising sharpness. 



Mr. Shadbolt prefers to find the necessary alteration for the foci 

 of different objectives. It seems almost a matter of regret that 

 opticians have not offered a special correcting eye-piece to be 

 employed with the one-inch or two-thirds objective, so that we might 

 obtain, at moderate distances, the advantage of the increase in 

 magnifying power, and at the same time preserve the unison of the 

 actinic and visual foci, and give a more perfect flatness of field ; but 

 in this case the eye-piece would require to be most correctly centered, 

 both with regard to itself and in relation to the axis of the object- 

 glass. Object-glasses with large angular apertures, unless possessing 

 great flatness of field, with perfect correction for spherical and chro- 

 matic aberration cannot be expected to supply the most useful 

 photographic objectives. 



In giving the amplification of an object, the simplest plan, 

 perhaps, is to divide the screen from the centre into inches and 

 tenths, to measure the size of the image and compare it with the 

 size of the object as given in the microscope with the micrometer, or 

 to substitute the micrometer for the slide, taking care to let their 

 surfaces coincide, and not to alter the correcting adjustment, as 

 with the high powers and single fronts the alteration in size is very 

 rapid. 



336. stereoscopic Photographs. Seeing the advantage derived 

 from the application of the stereoscope in viewing the dissimilar 

 images of large objects taken at varying angles, it was natural 

 to suppose that an effort would be made to produce stereoscopic 

 images of minute objects. Professor Wheatstone suggests in the 

 Transactions of the London Microscopical Society, for April, 1853, 

 a plan of procuring these at the necessary angles. He proposes that 

 the tube of the microscope should have an independent movement 

 of about 15, "round an axis, the imaginary prolongation of which 



