WITH THE MICROSCOPE. 37 



11 Test Objects," by Dr. Carpenter, "The Microscope and its 

 Revelations," pp. 141, et seq. 



02. Simple Method of Measuring Objects. The most simple and 

 efficacious manner of measuring objects is with the aid of the camera 

 lucida or neutral tint-glass reflector referred to before, 44. In 

 the field of the microscope is placed an ordinary micrometer, with 

 the lines separated by thousandths of an inch. Care being taken 

 that the instrument is arranged at the proper distance from the 

 paper, the lines magnified by a quarter of an inch object-glass are 

 carefully traced. The micrometer is removed and replaced by the 

 object whose diameter is to be ascertained. In pi. XIII, fig. 64, 

 both micrometer lines and objects are shown magnified by the same 

 power. The object is traced over the lines, or upon another piece 

 of paper, and compared with the scale by the aid of compasses. 

 The lines may be engraved upon a slate, or upon pieces of ivory 

 or cardboard, and their value affixed, so that any object may be at 

 once measured. We require of course a different scale for each 

 power. Such scales may be made on pieces of gummed paper, 

 and one of them may be affixed to every microscopical drawing. 

 Fig. 63 shows several such scales magnified by different powers. 

 Thus the size of every object delineated may be at once ascertained, 

 and the trouble of making individual measurements saved, while at 

 the same time the inconvenience of a long description of the 

 dimensions of various objects is avoided, than which nothing can 

 be more tedious or less profitable to the reader. 



In comparing the representation of the same object delineated 

 by different observers, it will be found that great confusion has been 

 produced in consequence of the magnifying power of the object- 

 glass not having been accurately ascertained, and an object said to 

 be magnified the same number of times by two authorities, is not 

 unfrequently represented much larger by one than by the other. This 

 discrepancy in most cases arises from the magnifying power of the 

 glasses not having been accurately ascertained in the first instance. 

 I cannot, therefore, too strongly recommend all microscopic 

 observers to ascertain for themselves the magnifying power of every 

 object-glass and to prepare, in the manner presently to be described, 

 a scale of measurement by which the dimensions of every object can be 

 at once ascertained. The plan of appending to every microscopical 

 drawing a scale magnified in the same degree as the object repre- 

 sented, supersedes the necessity of giving measurements in the text, 

 while it is free from any of the objections above referred to. 



63. On Ascertaining the Magnifying Power of Object-glasses. 

 I will now describe the method of ascertaining the magnifying 



