126 



PRACTICAL TREATISE ON THE MICROSCOPE. 



three inches in length, and five-eighths of an inch in diameter, 

 as seen in fig. 76, into the opposite ends of which are screwed 

 two lenses, a, c; a being a meniscus, and c a 

 planoconvex, and both having their convex 

 surfaces towards the eye-piece which is 

 situated in the upper part of the draw- 

 tube ; between them is placed a stop, b, with 

 a small hole in it. The use of this instru- 

 ment is similar to that of the same arrange- 

 ment of lenses in the eye-piece of a tele- 

 scope, viz., to cause the image of any 

 object to be seen in the erect or natural 

 position. The field of view is also greatly 

 increased, and an object as long as the three- 

 fourths of an inch, can be taken in at once 

 with the erector and a two-inch object-glass.. 

 By pulling out the draw-tube, and therefore 

 increasing the distance between the erector 

 itself and the object-glass, the magnifying 

 power of the instrument is increased, and by 

 pushing it in again the power is diminished ; 

 so that a microscope with a two-inch object- 

 glass and the erector can be made to take 

 in as much of a rule as three-fourths of an inch in length 

 when the draw-tube is only slightly pulled out; and when 

 the tube is drawn out to its fullest extent, it will magnify the 

 divisions on the rule so much, that one-sixth of the same object 

 alone, will fiU the whole field of view. 



The erector was first applied to the compound microscope, 

 represented by fig. 21, by Mr. Lister ; it is extremely useful 

 for taking in large objects, but more particularly for dissect- 

 ing, as heretofore the inversion of the object by the compound 

 microscope, entirely prevented any dissection being carried on 

 under any of the low magnifying powers; but, with the 

 erector, it can be done very readily. 



Lieberkuhns. — These are concave silvered specula, so named 

 from their iUustrious inventor; they are attached to all the 

 object-glasses, from the two-inch to the one-fourth; that for the 



