82 MICROSCOPES FOR SPECIAL PURPOSES. 



by the preference which the taste of the purchaser, or the nature 

 of the researches on which he may be engaged, may lead him to 

 entertain for one or other of the plans of construction which has 

 now been brought under review.* 



Microscopes for Special Purposes. 



Of the large number of Instruments which have been in- 

 geniously devised, each for some particular use, it would be quite 

 foreign to the purpose of this Treatise to attempt to give an account. 

 A few forms, however, may be noticed, as distinguished either by 

 their special adaptiveness to very common wants, or by the inge- 

 nious manner in which the requirements of particular classes of 

 investigators have been met. 



56. Prof . Beetles Pocket Microscope. — This instrument consists 

 of an ordinary Microscope-body, the Eye-piece of which is fitted 

 with a draw-tube, which slides smoothly and easily ; whilst its 

 lower end is fitted into an outer tube, of which the end projects 

 beyond the objective. Against this projecting end the Object-slide 

 is held by a spring, as shown in Fig. 40, being fixed (if necessary) 

 by a screw-clip. The coarse adjustment is made by sliding the 

 body through the outer tube which carries the object ; and the 

 fine adjustment by sliding the eye-tube in or out. The object, if 

 transparent, is illuminated either by holding up the Microscope to 

 a window or lamp, from which the rays may pass directly through 

 it, or by directing it towards a mirror laid on the table at such an 

 angle as to reflect light from either of these sources : if opaque, 

 it is allowed to receive direct light through an aperture in the 

 outer tube. The extreme simplicity and portability of this instru- 

 ment (which when closed is only six inches long) constitute its 

 special recommendation. Being fitted with the Universal Screw 



the most careful preparation of the object : hence it may be employed 

 with the same facility as an Objective of much longer focus ; but as its 

 Angle of Aperture is limited to 140°, it does not much surpass a good 

 1-Sth inch Objective in its power of resolving Diatom-tests. — The 

 l-25th-inch of Messrs. Powell and Lealand is an Objective of marvellous 

 perfection, and has proved itself admirably adapted to the higLest class 

 of Physiological investigations. The same Makers have even produced 

 a l-50th-inch Objective ; but thoxigh its power is double that of the 

 preceding, the Author is not aware that the increase has been found to 

 be attended with any decided practical benefit. 



* Several other Opticians may be named as makers of Microscopes 

 which deserve to rank in the First Class, on account both of then- Optical 

 and of their Mechanical excellence ; such are the instruments constructed 

 by Messrs. Baker, Collins, Crouch, Dallmeyer, Ladd, Pillischer, Swift, 

 and Wheeler. These are for the most part copied, with more or less of 

 modification in detail, from the models either of Mr. Ross, or of Messrs. 

 Smith and Beck ; very little that is original having been introduced. 

 The prices of these instruments, however, are usually from 10 to 20 per 

 cent, less than those of the corresponding instruments of the original 

 constructors ; and excepting with the highest powers, little inferiority 

 would be discovered in their performance, save by the most critical 

 judges. 



