WITH THE MICROSCOPE. 73 



12. Subject pieces of moist tea leaves, very thin sections of 

 potato and the peel of the potato, the skin or interior of an orange, 

 lemon or other fruit, a piece of rhubarb, cabbage, or other vegetable, 

 taking care that in all cases the pieces are small enough. They can 

 easily be subdivided with a sharp penknife. 



I strongly recommend the beginner to examine various specimens 

 of jam and preserved fruits. As these vegetable tissues have long 

 soaked in syrup, they have become exceedingly transparent, and are 

 admirably fitted for microscopical demonstration. The spiral vessels, 

 woody, and cellular tissues, can be obtained without any trouble, and 

 the minute structure of the different vegetable tissues can be most 

 clearly demonstrated. 



The thinnest possible sections can be cut with a sharp thin knife, 

 p. 45, from the firmest of these preserved fruits. The specimen 

 may be placed in a little syrup for examination. 



The action of syrup and glycerine will be more fully discussed 

 in part V. 



ISO. Precautions to be observed in working. And now I must 

 give a few words of advice to the young observer not to work too 

 long at a time or with high powers, or to have the object illuminated 

 more intensely than is necessary to enable him to see it clearly. To 

 avoid strain, the habit of keeping both eyes open during observation 

 should be acquired as soon as possible, and the observer should 

 observe sometimes with one eye and sometimes with the other. 

 Although the eye improves very much by practice, it may be seriously 

 damaged by straining it injudiciously. At first the observer should 

 work for half an hour only at a stretch, and if he finds that he is 

 not fatigued and external objects are seen quite distinctly, as to 

 form and colour, immediately the eye is removed from the micro- 

 scope, the period of observation may be gradually increased until it 

 reaches two or three hours a-day, but I think it unwise to work unin- 

 terruptedly for a longer time, and it is a good plan not to work 

 regularly every day, at least for the first year or two. With care 

 an eye which was at first weak may be inured to prolonged exertion 

 and employed for the greater part of life without damage. 



It is remarkable how little some persons suffer from microscopic 

 or telescopic observation, but it is quite certain that many cannot 

 work for long without great risk of seriously injuring their sight. 

 No general rules, therefore, can be given which should apply to all. 

 I have myself often worked with very high powers and with a very 

 brightly illuminated field, straining the eye to the utmost in the hope 

 of seeing more than was at first observable, and have kept this up 

 for some hours at a time, so far, I am happy to say, without any 



