700 Transactions of the Society. 



The " reflectors " and the " Gundlach " will be found in use to 

 " invert the image " ; this inversion is very troublesome, and if not 

 well understood, and met with certain devices, is apt to lead the 

 draughtsman into endless confusion. In practice when you use 

 one of these inverters you are compelled to sketch from the one 

 side of a slide of objects, and to fill in detail from the other, and as 

 a clever writer has pointed out with respect to this arrangement, 

 " the back and the front of an object are not always alike." 



The image got by a Wollaston prism is so excellent that this 

 instrument would always be used, were it not that the setting of the 

 Microscope in a horizontal position, the re-arrangement of the 

 light, &c., the dependent position of the eye while drawing, the more 

 or less cramped position, and other difficulties with respect to the 

 slipping of fluid preparations oblige one to employ more convenient 

 though perhaps in other respects less satisfactory appliances. Of 

 the prisms used at the ordinary observing angle of the Microscope, 

 Gundlach's, as I said, inverts the image, and 1 am sorry to say that 

 the Zeiss prism, though it is quite satisfactory in my hands, in 

 most respects, projects the image so far forward as almost to come 

 upon the stand of the Microscope, and so practically cramps the 

 position of the drawing paper or board ; I therefore seldom use 

 it. I like the Beck prism, and I make perhaps more use of it 

 than of any other, as the light transmitted, the field, and the sight 

 of the pencil are all satisfactory. 



The Schroder prism just invented has several points of 

 excellence, which will win appreciation. It shares with the 

 Wollaston the rare quality that the pencil is seen with equal 

 distinctness in all parts of the field, and that there is no apparent 

 change in the position of the point of the pencil from involun- 

 tary oscillation of the head of the draughtsman. I am only sorry 

 to be obliged to say, that I find the usefulness of this neat 

 little instrument is much limited by the very small amount of light 

 it transmits from the object under the Microscope ; entailing the 

 condensation of such a body of light by " racking up," when 

 anything like a high power objective is used, as rather to strain 

 the vision and make anything like detail too much a matter of 

 guess. I am encouraged to hope that this condition may be 

 susceptible of modification, and in such case the Schrdder prism 

 would not leave much to be desired. 



I may just say, that while not appreciating the Abbe prism 

 used for the purpose for which it was constructed, I recognize a 

 most valuable quality which it possesses, for copying drawings and 

 engravings of small area, either of the size of the original or with 

 a slight magnification at will. I think I can see a considerable 

 future for this prism in certain branches of the fine arts. 



