FIRST PRINCIPLES 41 



is measurement." Much of the credit of its advance is due 

 to the instrument-maker. 



A list of the instruments of precision which are at the 

 service of workers in various branches of science, with a 

 comparative statement of their delicacy, would be of great 

 interest. But such a list would be misleading unless elab- 

 orate explanations were given of the conditions under which 

 they can be used with their maximum of sensitiveness. 

 For example, a microscope fitted with an objective of -jVinch 

 focal length and an eyepiece multiplying twelve times 

 will give a magnification of 3,000 diameters. Yet it is rarely 

 that so high a power can be usefully employed. Clearness 

 of definition is to a certain extent sacrificed to magnifica- 

 tion ; the loss of "penetrating power" restricts the use of 

 such a combination to sections of the extremest thinness 

 and most vivid staining. 



Delicate apparatus for testing and examining objects 

 exact great nicety in the preparation of the objects for 

 examination. In no case is the recent improvement in 

 method more conspicuous than in the preparation of sections 

 for the microscope. Thirty years ago the histologist placed 

 the specimen of which he wished to prepare a section 

 between two pieces of cork or elder-pith, and cut it with a 

 razor which he held in his hand. Now he has microtomes 

 of many patterns to choose from. He may embed the 

 object in celloidin, soak the mass with water and cut it 

 frozen ; or he may embed it in paraffin and cut it on a 

 riband-microtome a section-cutter, so named because the 

 slices of paraffin are caused to adhere to one another at 

 their edges, making a continuous riband. The steady hand 

 upon which the microscopist used to pride himself is no 

 longer required. A laboratory boy turns a handle, or the 

 machine is connected with some form of motor, and sections 

 fall away from the razor in a band of paraffin which can be 

 mounted almost automatically on glass sides. Nor is it 

 necessary to mount every section. The machine will select 



