THE SOLAR MICROSCOPE. 



By the interposition of such, a medium, the object may be pre- 

 vented from receiving any increased temperature whatever. 



It happens that water, which is the most convenient medium 

 for this purpose, is very imperfectly pervious to heat, and is ren- 

 dered almost completely athermanous by dissolving in it as much 

 alum as it is capable of holding in solution. The object, therefore, 

 is perfectly protected from the effects of heat, by placing between 

 the slider and the condensing lens a cell, consisting of two parallel 

 plates of glass, fixed at about an inch asunder, and filled with such 

 a saturated solution of alum. The light intercepted by this is alto- 

 gether inconsiderable, while the whole of the heat is stopped by it. 



6. The magnifying part of the solar microscope consists of an 

 achromatic lens, or combination of lenses, of very short focal 

 length ; this being brought before the object, at a distance from it 

 a little greater than its focal length, will produce a highly mag- 

 nified optical image of the object, upon a screen placed at a proper 

 distance before it. 



In the case of the magic lantern, it is not indispensable to incur 

 the expense of achromatic lenses, and even the expedients to cor- 

 rect the spherical aberration are but little attended to. The 

 magnifying powers used in that instrument not being great, and 

 the objects exhibited not requiring extreme accuracy of delinea- 

 tion, the expense which would be incurred in producing large 

 lenses free from the aberrations is not necessary. But in the case 

 of microscopic objects, where great magnifying powers are applied, 

 lenses in which the aberrations are not corrected would produce 

 images so confused and indistinct as to be altogether useless. 

 Achromatic combinations, therefore, in which the spherical aber- 

 rations are also corrected, are in this case indispensable. 



As in the magic lantern, the same lenses may be applied, so as 

 to produce different magnifying effects. If the distance of the 

 lenses from the object were so great as twice their focal length, 

 the image would be projected upon the screen at a distance in front 

 of the lens also equal to twice its focal length, and would in that 

 case be exactly equal to the object, and consequently there would 

 be no amplification at all. As the lenses, however, are moved 

 nearer to the object, the distance at which the image would be 

 formed and its magnitude would be increased, and this increase 

 would go on without practical limit, until the distance of the lens 

 from the object would become equal to its focal length, in which 

 case the image, having been enlarged beyond bounds, would alto- 

 gether disappear. 



In practice, therefore, the focus of the lens is brought to such a 

 distance from the object, that the image upon the screen shall 

 have a magnitude sufficient for all the purposes of exhibition. It 

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