WITH THE MICROSCOPE. 255 



When the photographer employs these objectives, more especially 

 the low and middle powers, he generally finds that either his prepared 

 sensitised plate must be moved further away from the plane at which 

 the best visual focus was found, or else he must withdraw his objective 

 a slight distance from the object, and bring the chemical focus to its 

 compensating point for the amount of " over correction " that has 

 been given to it by the maker. This is not a fixed sum, and may 

 vary in different object-glasses furnished by the same optician, when 

 of equal magnifying power, or even ground on the same tools. In 

 the construction of some of the lower powers a plan has been adopted 

 which, at the same time that it does not detract from their optical 

 perfection, places the chemical variance at its lowest mean. In the 

 higher powers, as from -i-th upwards, the difference between the visual 

 and chemical foci is so small that it is seldom regarded, except in 

 the most delicate work ; but here the disturbance occasioned by the 

 cover of thin glass placed over the object, requires the adjustment 

 between the two posterior combined set of lenses, and the anterior 

 pair, triple or single lens, to be made with the greatest nicety, as has 

 been strongly advocated by Dr. Wilson. It is not possible to deter- 

 mine beforehand the amount of alteration in focus needed, and a 

 series of trials will be necessary to establish what adjustment is 

 requisite. The best plan is to select an object that has a slight 

 thickness, with parts at a distance from one another, lying in three 

 or four different planes. Set the objective to the best focus in the 

 microscope, then place it in the camera ; focus sharply for the part 

 of the object nearest, and in the negative which is taken, observe if 

 this part corresponds in definition, or if not, which plane of the object 

 appears the sharpest. Let us suppose the furthest plane ; then 

 observe, by re-focussing, how many divisions of the milled-headed 

 screw have been turned through to bring this part into as perfect a 

 focus as was originally the nearest plane. This will give the variation 

 for that objective under similar circumstances, and should be noted. 

 If employed with the shallow eye-piece, to increase the magnifying 

 power, with the loss of some definition, a different adjustment may 

 be required. Mr. Shadbolt undertook a series of experiments for his 

 objectives, of Messrs. Smith, Beck, and Beck's make, when he em- 

 ployed artificial light, and which he gives as follows : 



The ii inch object-glass to be withdrawn ^th of an in. 



,, 



A t-h -! __ 



1 "o " 1000 



These can only be regarded as guiding marks for others. To 

 obviate this great inconvenience, Mr. Wenham, to whom we owe 

 much for the perfection of the binocular microscope, with his usual 



