204 OPTICAL PROJECTION 



only some of the coarsest can be shown even imperfectly by 

 the lime-light, as the stopping down of the light to get crispness 

 impairs the brilliancy so seriously. A good \ or \ amplified 

 does the best work with most of them. 



With polarised light, besides the crystallisations already 

 mentioned, all mineral sections which polarise exhibit ex- 

 cellently ; also fish scales and most organic objects. 



These details, by no means complete, will be sufficient to 

 show how wide is the range of the projection microscope 

 even with oxy-hydrogen illumination. 



102. Living Objects. Pond life is always a popular sub- 

 ject, and the vast mass of objects can be shown with ease 

 in the projecting microscope. The larger beetles and larvaB 

 are not really microscopic, but require a large trough in the 

 ordinary stage of the lantern. For smaller objects a variety 

 of glass troughs must be provided, which can be procured 

 for Is. each upwards. The troughs should be so thin as 

 only to allow the creatures to move freely in the same plane ; 

 for free movement they must not be tighter than this. Often, 

 for more minute examination, as of the internal organs of a 

 water-flea, it is necessary to check the power of movement 

 by somewhat compressing the animal. This can be done by 

 using a live-box, or by slightly forcing the creature into a 

 rather thin trough with the end of a sable pencil, or by 

 placing it with a drop of water on one of the glass slips made 

 with a concavity on one side, and covering it with a thin 

 glass, held on by capillary attraction. Very small animals, such 

 as rotifers, are often best dealt with by placing them in a drop 

 of water on a plain glass slide, and covering them with a thin 

 slip; or if the pressure would be too great thus managed, 

 the creature may be encircled with a bit of cotton thread. 



Glass troughs are very easily made to any thickness, by 

 taking an ordinary 3x1 slip, and a thin piece of cover-glass, 

 and cementing between them, with dried Canada balsam 

 dissolved in benzol, the half of a vulcanised rubber ring. If 



