FIFTH AND SIXTH CLASSES OF MICROSCOPES 237 



that need be desired. The stage has the horseshoe -shaped aperture. 

 The sub-stage fitting, as shown in the illustration, may be turned 

 aside out of the optical axis, and a compound sub-stage may be made 

 with the instrument if desired. Throughout, the working parts are 

 sprung, and wear may be compensated by adjusting screws. We 

 cannot speak too highly of the enterprise and skill shown in the design 

 and manufacture of this instrument ; and yet the student will find 

 that, good as it is, it is one of the least costly instruments of its class. 



There is a microscope manufactured by Messrs. Zeiss, known as 

 ' Stand VI. A,' which comes to about the same cost as the above, and 

 which we illustrate in fig. 190. It is of course a strictly Continental 

 form, having a fixed stage 3J ins. square. The coarse adjustment is 

 by rack and pinion, and the fine adjustment is the usual micro- 

 meter screw of these makers. The stand is inclinable, and it is 

 provided with mirrors and a cylinder diaphragm which slides in a 

 sleeve fixed below T the stage capable of receiving the illuminating 

 apparatus. It is, of course, made with the accuracy and good 

 quality of workmanship for which this firm is noted. 



Fifth and sixth classes of microscopes arc made by the best 

 makers, and it is a little notable that the best of these classes was made 

 by the late Hugh Powell, whose maxim was that a microscope with 

 only a good coarse adjustment was to be preferred to one having an 

 indifferent fine adjustment with a sliding tube for the coarse adjust- 

 ment. 



This stand is of cast iron, with a flat tripod, having a single 

 pillar to which is jointed the Jackson body. The focussing is 

 admirable ; the stage is of an excellent form, being 4^ x 3^ inches, 

 and is supplied with a beautifully made sliding ledge, which will 

 move easily and firmly with pressure from one side only. 



The stage is fastened to the upper side of two brackets which 

 are cast in one piece with the limb ; on the under side of these 

 brackets there is another plate which holds the sub-stage tube. 



This instrument is supplied with large plane and concave 

 mirrors ; and, considering that it constitutes a sixth class of 

 microscope, has very much in its favour as a secondary instrument 

 for the work-table. Like all these makers' instruments, the feet are 

 plugged with cork ; and we know of some of these microscopes that 

 have been in use for forty years, and are still the trusted 'journey- 

 men ' instruments of mounters and other workers of various orders 

 in many departments of microscopy. 



Some of the modern forms of these two classes of microscope 

 deserve, on behalf of beginners with limited means, some considera- 

 tion. A thoroughly good but extremely simple microscope of the 

 fifth class is made by Watson and Sons ; it is illustrated in fig. 191. 

 It was designed for educational purposes ; the workmanship is of the 

 finest quality, but the instrument is not provided with a fine 

 adjustment ; it relies on a very perfectly made diagonal rack and 

 pinion coarse movement. From practical use we can speak in the 

 highest terms of the delicacy of this focussing arrangement, with 

 which we have with ease used powers up to ^ inch, and often have 

 used it with a -fa-in. objective. The stage is large, the body has a 



