WITH THE MICROSCOPE. 35 



the magnifying power of the combination of lenses he employed, and 

 should append a scale magnified by the same combination, See 64. 

 But the reader must not conclude that I am insensible to my 

 own shortcomings in these and many other matters. I am conscious 

 that every drawing I have published might have been, and ought to 

 have been better. I can only hope therefore that the desire for 

 seeing our work useful to each other and to our successors, as well as 

 to ourselves, will be received as a sufficient apology for these remarks. 



ON MEASURING OBJECTS AND ASCERTAINING THE MAGNIFYING 

 POWER OF OBJECT-GLASSES. 



Most of the larger and complete microscopes are furnished with 

 special micrometers, but the simple method of measuring objects, 

 presently to be described, to a great extent supersedes more 

 expensive arrangements. It will be well for me, perhaps, in the 

 first place, to describe briefly the different forms of micrometers 

 in use. 



58. The Cobweb Micrometer, originally applied to telescopes by 

 Ramsden, its inventor, can be fitted to the upper part of the body 

 of the microscope. A fixed cobweb crosses the field of view, and 

 parallel to this is another cobweb thread capable of being brought 

 near to, or separated from the first, by turning a milled head, to 

 which is attached a graduated circle. The value of each degree on 

 the circle is ascertained by placing an object of known dimensions, 

 as the stage micrometer graduated to thousandths, under the object- 

 glass, and ascertaining the number of degrees on the screw which 

 corresponds to the i-ioooth of an inch. From these data a simple 

 table may be constructed, and the diameter of any object can be 

 readily ascertained by bringing one side of it up to the fixed line, 

 and causing the moveable line to touch the opposite. If we ascer- 

 tain the value of the degrees as marked upon the circle when the 

 lines are separated at the proper distance, we may estimate directly 

 the diameter of the object. The older observers used to measure 

 objects by means of very delicate wires, separated from each other 

 by certain known distances, placed in the focus of the eye-piece, or 

 by employing points, one of which could be moved from, or towards, 

 the other by means of a screw. 



59. Jackoii's Eye-piece Micrometers. Mr. Jackson arranged a 

 micrometer slide in the eye-piece so that it could be brought over 

 the magnified image of the object by means of a screw. 



GO. stage Micrometers. Within the last few years, lines, sepa- 

 rated from each other by certain known but very minute intervals, 



D 2 



