135 



When Mr. Lister undertook, in conjunction with Dr. Hodgkin, to 

 measure the human blood-discs, and to determine which of the very 

 discordant statements of anatomists was most entitled to confidence, 

 he used the following method. 



By means of the camera lucida he first made a sketch of the 

 blood-discs ; and then, without altering the distance of the paper, or 

 the optical arrangements of the microscope, he placed a slip of ruled 

 glass on the stage in place of the object, sketched it on the paper, 

 and used it as a scale to measure the drawing that he had previously 

 made. The object and the micrometer being both magnified to the 

 same extent, their images will necessarily bear the same relation 

 to each other as the bodies themselves. The method is, therefore, 

 correct in principle, and should always be practised when drawings 

 are made, as preferable to the common mode of merely stating the 

 magnifying power used. 



Micrometric measurements then are made by comparing the magni- 

 fied image of an object with the similarly magnified image of a scale 

 of known dimensions ; and we have next to Consider /Che readiest and 

 simplest means by which this may be effected ; taking care to attain 

 the utmost accuracy that the subject admits. 



Although Mr. Lister's mode is capable of a good deal of precision, 

 yet, when a drawing is not otherwise required, it is not the most 

 ready method that has been contrived. If a micrometer be placed in 

 the focus of the eye-glass, it will serve as a common measure by 

 which an object on the stage, and another micrometer in the same 

 situation, may be compared : and it has this among other advantages, 

 that when we have ascertained with each of our object-glasses the 

 relation which exists between the two micrometers, that on the stage 

 may be dispensed with. 



The micrometer generally used with astronomical telescopes con- 

 sists of two parallel wires (or spider's webs), one fixed, and the other 

 movable by means of a screw, the head of which is divided into one 

 hundred parts. This, with a simple contrivance for reading the num- 

 ber of turns made, enables us to ascertain the distance of the wires to 

 the hundredth of the turn of the screw : and, as the latter has 

 commonly about one hundred threads to the inch, it should mea- 

 sure the magnified image of an object to the ten-thousandth of an 

 inch. 



With an object-glass of one-eighth of an inch focus, and a body of 

 the usual length, the image will be amplified about eighty times, 

 exclusive of the power of the eye-piece ; and, therefore, a quantity as 



