188 .SELECTION AND USE 



The operation which we have thus briefly described as appli- 

 cable to the potato, is required for a great many other materials ; 

 for whenever a substance is to be examined under any except 

 the very lowest powers, it is absolutely necessary to obtain it in 

 pieces as thin as possible, so that the light may readily pass 

 through them, and it is in general requisite to increase their 

 transparency either by immersing them in a fluid, or by some 

 other means. In preparing objects for the microscope, our 

 aim is in general to examine either the ultimate structure ol 

 the substance under investigation, or the arrangement of its 

 different parts; and the processes which are most available for 

 this purpose may be classed under three heads: 1, Mechanical, 

 such as section-cutting, dissection and injection; 2, Chemical, 

 such as the use of iodine for detecting starch; of alcohol for 

 hardening certain structures; of coloring substances for stain- 

 ing germinal matter, etc. ; 3, Optical, such as the action 

 whereby certain liquids change the transparency of some ob- 

 jects. Of some of these processes, such as injection, staining 

 and the extended use of chemical tests, elaborate descriptions 

 would be required in order to enable the student to carry them 

 out with success, and we must refer him to the works of Beale 

 and Frey, which are very complete on these points. 



Thin sections of any soft substance are easily made with a 

 very sharp knife a good razor being probably the best availa- 

 ble instrument. For work in the higher departments of micro- 

 scopy, and for the preparation of fine objects for sale, special 

 instruments known as section-cutters are employed, but for the 

 ordinary work of investigation, they are not absolutely neces- 

 sary though very convenient. Using a good sharp razor, it is 

 an easy matter to shave off any soft substance a wedge shaped 

 piece, the edge of which thins off to nothing, and which pre- 

 sents in its different parts all varieties of thickness, so as to afford 

 a perfect opportunity to study the object under examination. In 

 this way, which is known as the " free-hand" method, suitable 

 sections of most animal and vegetable substances may easily be 

 prepared, and the student will be surprised at the dexterity 

 which a little care and practice will confer. 



For cutting sections of very soft tissues a special knife, known 

 as Valentin's knife, has been invented. It consists of two 



