xiv INTRODUCTION. 



wish to become acquainted with the microscope as an optical instrument, first to study the 

 general laws of optics, which may "be done through the medium of any of the works or 

 treatises on Natural Philosophy as the article ' Optics' by Ilerschel in the Encycl. 

 Metropolitan, Brewster's 'Optics,' Lloyd's 'Manual,' Deschanel's 'Optique,' Verdet's 

 1 Lecons d'Optique Physique,' Nageli and Schwendener's ' Mikroskop/ ' Ganot's t Physique ' 

 (transl. by Atkinson), or Rodwell's ' Dictionary of Science.' Perhaps the second work is the 

 best for the general reader ; it is an old standard work, but greatly behindhand in regard 

 to the use of the microscope. They may then proceed to the application of these laws to 

 the various optical parts of the microscope. This will be found in the works enumerated 

 at the end of the Introduction. 



We must not, however, omit a notice of the principles which should guide in the selec- 

 tion of a microscope and the accessory apparatus, because a large number of microscopes 

 are at the present day sold, frequently at no mean cost, which, although well calculated 

 to afford amusement, are utterly valueless for the purpose of scientific investigation. To 

 those to whom money is no consideration, we may recommend with safety, as the best 

 which can possibly be procured, such as are manufactured by Ross, Smith and Beck, or 

 Powell, of London. These makers have a thorough knowledge of the instrument, and a 

 reputation at stake ; hence there is little occasion to test their instruments. But it may 

 happen that a person may not wish to expend so much money as the purchase of these 

 instruments requires, may wish to procure a foreign instrument (and these are cheaper), 

 or may meet with one second-hand. A word or two may then be of service in guiding 

 them in their choice ; for a microscope may look very well and very handsome, yet be 

 worth but little. It must, however, be borne in mind that there is much room for opinion 

 in these matters j for according to what any one has been accustomed to, or according to 

 prejudice arising from what he may have heard a supposed authority say, so will an 

 instrument or a piece of apparatus be regarded as requisite or of importance, or not. 

 Our statements rest upon our own experience in the long-continued use of the instrument, 

 and as such they must be taken. 



First, it may be remarked that the microscope is usually regarded as composed of the 

 stand, body, stage, eye-pieces, and object-glasses : and the object-glasses are generally sold 

 separately ; for by means of an " adapter " they can be applied to any microscope. 



In regard then to the stand, body, c. : the stand should be firm, and so heavy and it* 

 feet so arranged, that the instrument cannot be easily overturned. 



The body, whether the microscope has one body only, or is binocular, should be about 

 8 or 10 inches in length j in many of the foreign and cheap English instruments the body 

 is short, and the eyepieces are adapted accordingly j but this adaptation is decidedly 

 objectionable. 



Whether the microscope shall be binocular or not must be a matter of opinion. In the 

 binocular microscopes there are two bodies and two eye-pieces, the rays of light just above 

 the object-glass being divided by a refracting prism into two portions, one of which passes 

 through each tube; in this way the stereoscopic view of objects is obtained. The 

 binocular arrangement is an additional expense ; it can be added to any microscope j but 

 any binocular microscope can be used as a single-bodied instrument (See BINOCULAR). 



The microscope should be so constructed that the body can be inclined at any angle 

 desired, so that the observer may examine objects while sitting. Many persons, however, 

 prefer to use the microscope with the body placed perpendicularly; and when chemical 



