108 Dr. Goring on the new Microscopes^ Sfc, 



further from the shallowness of their curves : it follows therefore, 

 that if a certain angle of aperture is just as effective and 

 valuable in the one as in the other, the reflector must inevitably 

 have the advantage over any single achromatic, at all events, 

 ■with those objects which require a very large one, and 

 accordingly this is found by experience to be the case. 

 Thus the lines on the feathers on the wing of the Papilio 

 Brassica3, which are at best but barely visible with the 

 single 0.3 achromatics, with an aperture of 0.15 are most 

 distinctly and satisfactorily demonstrated by the 0.3 focus 

 metals with an aperture of 0.2 : the same may be said of the 

 interlaced lines and lozenges on the scales of the Podura, that 

 most valuable object lately discovered by Mr. T. Carpenter. 



Opticians have lately been diligently exercising their powers 

 in stringing object-glasses together — instead of getting their 

 power and aperture in one glass, they obtain it bit by bit 

 in detached portions. This practice has been carried much 

 too far, but it must be confessed that when two objec- 

 tives are combined together in a fitting manner, they tread 

 very closely on the heels of a reflector in most respects, and 

 fully rival it in dulness and darkness, from the multiplicity of 

 the refractions employed, though by no means in the singleness 

 and simplicity of their optical operation. Two triple aplanatics 

 or three double ones combined, give, I think, as dark an image 

 as a reflector of the same angle of aperture ; in addition to 

 which there is a want of clearness and vivacity in the picture 

 which certainly cannot be laid to the charge of that of a good 

 reflector. It is a known fact that errors by reflection are six 

 times greater than those by refraction ; but if we have twelve 

 curved surfaces to manage in a glass microscope, its errors 

 will be most probably about equal to those of a catadioptric 

 constituted by two reverberations like that of Professor Amici. 

 Those who know how to put optical instruments out of focus, 

 and look into their defects, will be able to estimate more nicely 

 the distinctions between reflectors and refractors both single and 

 composite : for my own part I think it an indisputable fact, that 

 no single achromatic can be put in competition with a reflector, 

 (setting aside the secondary spectrum, always incorrigible by the 

 best arrangements,) unless the apertures employed in both are 



