xii INTRODUCTION. 



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 Herschel in the 

 Encycl. Metropolitan ;Brewster's 'Optics;' Biot's 'Manuel,' &c.; the ' Natural Philosophy ' 

 of the Society for the Diffusion of Useful Knowledge ; Lardner's ' Natural Philosophy/ or 

 Mrs. Somerville's ' Connexion of the Physical Sciences.' Perhaps the second work is the best 

 for the general reader; it is quite recent (1853), but greatly behindhand in regard to the 

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

 the various optical parts of the microscope. This may be found to some extent in the useful 

 work of Quekett on the Microscope, in which the various kinds of microscopes and accessory- 

 apparatus are figured and their action described, with lists of objects of interesting appear- 

 ance, &c. ; Brewster's Treatise on the Microscope; art. 'Microscope' in the Penny 

 Cyclopaedia by Ross. There does not, however, exist a w r ork in which this is satisfactorily 

 done. The ' Observateur,' &c. of Dujardin is an admirable work, in many respects the 

 best ever written ; the ' Micrographia ' of von Mohl is greatly esteemed in Germany. 



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

 selection of a microscope and the accessory apparatus ; because a large number of mi- 

 croscopes 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 investiga- 

 tion. 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 Smith and Beck, Ross, 

 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 purchase 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 room for much opinion in these 

 matters, for according to what any one has been accustomed to, or according to prejudice 

 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 so. 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 

 object-glass or glasses, and the stand, body, stage, eyepieces, &c. ; 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 its 

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



The body should be about 8 or 10 inches in length ; in many of the foreign instruments 

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

 objectionable. 



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 

 reagents are to be applied this position is essential ; but when long-continued examination 

 of an object is required, it becomes very painful and fatiguing to keep the head in the 

 position which the perpendicular position of the body requires. Moreover, as in a microscope 

 with the joint or arrangement by which the body can be inclined, the body can always be 

 placed perpendicularly, the joint is decidedly advantageous. Again, it is almost essential 



