OF THE UNITED STATES. S7y 



curacy and easy manipulation, and I obtained it in as perfect 

 a manner as possible, by nieans of tbe following arrange- 

 ment : — 



This part of the apparatus is a microscopic arrangement, 

 ofwiiici) I'late 111. fig. i, presents a vertical section, of ai)Out 

 thefouith part of their real size, showing all the screw mo- 

 tions in the direction of the three rectangular ordinates. 



A compound microscope, aa, about seven inches long. 

 is kept in a vertical position, l)y passing tlirough two ho- 

 lizontal plates, bb, projecting from two columns, ee. It 

 can be raised or lowered to bring it to the proper focal dis- 

 tance, and tlien hold fast by the screws, dd, which press the 

 spring of tlic circular part of tlie plates, bb, together. 



The object glass of this microscope is formed of two halves 

 of lenses of ditterent foci. The one half will Ijring the 

 image of a cobwel) tlircad stretched over the end of the bar 

 at €, about three inches fiom the lens, to the focus of the eye 

 lenses: the other half lens will bring to the same focus the 

 image of the rectangular crossing of two lines traced on a 

 small plate of ivory, h, which is screwed to the middle of 

 the thick brass plate, ii, at about six inclics from the object 

 lens, and adjusted in the collimation line of the microscope. 

 The ends of the bar have, through the middle of their breadth. 

 a semicircular opening, to admit the cobweb to l)e stretched 

 across it, and to admit more light from the point h, to come 

 to the microscope, as seen in its natural size in the horizon- 

 tal section at g. 



In using the a|)[)aratus, the images above n^entioned are to 

 be brought in contact as in a reflecting mstrument, cither by 

 the screw of the microscope, or that of the bar, as the case 

 may require. As the coI)vvcb thread at the end of tlie bar 

 is perpendicular to the direction of the base, it is best to 

 place the rectangular crossing lines on the ivory plate, so as 

 to make angles of 45'^ with the direction of the base. The 

 contact will then be effected l)y makinii; the image of the cob- 

 wel) bisect the right angle thus formed. The mierosropes 

 ihemselves arc to l)c jjlaced <:o that the line dividitig the two 



