THE MICROSCOPE STAND 95 



any of the adjusting milled heads. The only other portion of 

 the instrument by which it may be lifted is the base or pillar. 

 Valuable instruments may be badly damaged if lifted carelessly. 

 Adjustment slides may be ruined, pinions and screws bent, or the 

 entire instrument may be dropped if these instructions are not 

 followed. 



^A suitable size for the mirror of a microscope depends upon Minor, 

 how far it is placed below the stage ; a large mirror close to the 

 stage is of no advantage. A 2-inch diameter mirror 3J inches 

 from the stage will converge a beam of light at approximately 

 30°, and this is more than is ever required. When a substage 

 condenser is employed, the mirror need not be much larger than 

 the back lens of the condenser, which never exceeds IJ inches. 

 A 2-inch mirror is, therefore, more than sufficient for all ordinary 

 types of microscopes. A mirror should preferably be on a fitting 

 by which its distance from the stage may be varied. It is not only 

 of advantage for focussing the concave mirror, but enables very 

 long apparatus to be used in the substage by sliding it farther 

 from the stage than its usual position. It is convenient that it 

 should be capable of swinging to one side for inserting substage 

 apparatus or for using light direct from a source of illumination, 

 but in use it must always be placed in the axis of the instrument. 



All the microscopes illustrated in this book possess the features 

 here described as being of importance ; the following brief notes 

 explain their special characteristics. 



Except the special metallurgical and petrological microscopes, 

 all the instruments illustrated are suitable for every branch of 

 work. The highest class of research work calls for a mechanical 

 stage and the best substage adjustments. The rack and pinion 

 adjustment to the drawtube and the rotating stage are of con- 

 siderable advantage, but are not essential. 



The Standard London Microscope is illustrated in six forms, standard 

 The first three of these forms are the same except as regards their ^^'^oscopes. 

 substages. These microscopes fulfil the conditions given in 

 previous pages as to the essential features which a serviceable 

 microscope must possess. They have also many smaller advantages 

 and refinements. The base consists of an iron casting of suitable The base, 

 weight to give rigidity to the instrument, encased in a covering 

 of vulcanite which gives it a durable finish. It is of such a spread 

 as to prevent the instrument from tipping, and is also made of 

 such a shape that it can easily be fixed down to a bench when used 

 for photomicrography when the instrument is used horizontally. 

 This may be done with advantage, as it prevents the microscope 

 from being moved during the process of attaching the camera. 

 The joint of the instrument is stopped at the exact vertical and 

 horizontal position. The stage consists of a brass core completely The stage. 

 embedded in vulcanite. This method is more satisfactory than 

 the usual method of fixing on thin vulcanite plate on the top of 



