OF ARTS AND SCIENCES. 327 



" But in examining transparent or translucent objects, which are to 

 be seen by transmitted light, and which constitute by far the most im- 

 portant class of objects for study with the higher powers, it is usual to 

 employ reflected light ; a mirror, plane or concave, being commonly 

 used for this purpose. The earlier microscopists often used direct 

 light, sometimes pointing their tubes to the sky, sometimes employing 

 a lamp, as in the instrument of Philip Bonnani, figured by Chevalier 

 and by Quekett, in their Treatises on the Microscope. Both these au- 

 thors mention the use of direct light, and give figures of the method 

 of employing it ; Chevalier representing a candle, and Quekett an Ar- 

 gand lamp, as the source of illumination. Many very delicate objects 

 are said by the latter author to be seen to the greatest advantage by 

 this kind of light. A late writer in Silliman's Journal recommends its 

 use in instituting comparisons between different lenses. It was used 

 by Mr. Spencer in the trial of instruments I have referred to, he hold- 

 ing in his hand a common lamp, with one wick picked down, behind 

 the stage, while we examined various objects with the higher powers. 



" The fact, that so simple a method of illumination proved sufficient 

 to define very delicate tests, led me to make trial of direct light as a 

 substitute for other methods. But none of the instruments made at 

 present being well adapted for its employment, I was led to contrive the 

 new model, which I now offer for the examination of the Academy, 

 and a figure of which accompanies this communication. This instru- 

 ment is constructed with particular reference to dispensing with all 

 reflectors, but can be used with excellent eifect with a prism, wlien the 

 light of the sky cannot be otherwise conveniently reached, as at a 

 window opening on a narrow street. The mirror, with its two reflect- 

 ing surfaces, glass and quicksilver, is got rid of, and with it several 

 sources of error and imperfection are removed. 



" The points I kept in view in the construction of this instrument 

 were these : a fixed horizontal poshion ; to dispense with machinery 

 as far as possible ; to employ the cheapest material. 



" Direct light requires a vertical stage ; a vertical stage implies a 

 horizontal tube, and permits the use of a horizontal support for all 

 illuminating contrivances. The power of gravity is therefore substi- 

 tuted for the ordinary adjustments of the parts above and below the 

 stage. It is an easy matter to arrange a service of plate in all posi- 

 tions and relations on a table, as it commonly stands, but not so if it is 

 inclined at an angle of forty-five degrees, or placed vertically. In this 



