OBSERVING AND MANAGING ENGISCOPE. 191 



times occur; but I have made most ample provision for 

 presenting all sorts of objects in any position, without re- 

 gard to their gravitation, by means of the aquatic wet and 

 dry glass boxes, and the slider-holder, &c. I think that 

 the practice of poring downwards (the ordinary way of 

 observation with microscopists of the old school,) is pe- 

 culiarly detrimental to the head and eyes, having a great 

 tendency to determine the blood to them. We never 

 hear of astronomers, ivho are perpetually looking upwards, 

 having their sight injured, like observers with microscopes, 

 though they have to look, if possible, still more stead- 

 fastly and intently than the latter, with the highest 

 powers. I had nearly forgot to mention a circumstance 

 very necessary to be attended to in the management of 

 the mirror; it is this, — to keep the milled head, r, inva- 

 riably outwards. It is not made to travel completely 

 round, nor is it necessary ; for every possible position 

 required for practical purposes can be attained withou t 

 its traversing more than the half of a circle, by reversing 

 the superior or inferior end of the ellipse, according to 

 circumstances. Thus, supposing there were two win- 

 dows in an apartment; one on the right, and the other 

 on the left of the instrument, it would command either, 

 without revolving on its inferior axis, merely by being 

 tilted on the right or left side. 



Way of viewing Crystallizations of Salts and various 

 Chemical Actions. — Use the mounting (No. 2); screw on 

 the lengthening piece, "i, and attach the diagonal boot 



