17^ MICROSCOPIC ILLUSTRATIONS. 



Fig. 15 represents the lengthening tube, 'i, with the 

 direct boot, 7c, drawn over it. The lengthening piece 

 screws on in place of the object-glass, which is again 

 attached to its extremity. This piece, from its narrow- 

 ness, will generate false light; it must, therefore, be lined 

 with black velvet, which is, perhaps, the best substance 

 to stifle it when generated. 



Figure 16 is the diagonal boot. It is constructed in the 

 same manner as the other, but has a plane metallic specu- 

 lum or rectangular prism of glass placed at'/, to reflect 

 the rays received at right angles. It is necessary that 

 the piece of plate-glass which is affixed to the extremity 

 of the cones of these boots should be very perfect, and 

 set in truly parallel. 



SirD. Brewster first conceived the project of causing an 

 aplanatic objective to be so constructed that it should be 

 capable of acting in w r ater instead of air (see his Treatise 

 on New Philosophical Instruments) . I am afraid it would 

 be very difficult to make a naked object-glass water-tight, 

 and at the same time to adjust it perfectly ; and after all it 

 would only serve to act in water, and would be useless out 

 of it. The present arrangement really seems to answer 

 every practical purpose completely well, though cer- 

 tainly Sir David's plan is the more scientific way of doing 

 the thing. 



Figure 17 is a cup or silver speculum, which slides 

 over the end of the object-glass. 



Figure 18 is a shade to slide over the lengthening 

 piece, 'i, figures 15 and 16. 



