Dr Goring on Monochromatic Light, SfC. 73 



to the body, so as to approach very near to the object-glass, 

 (in which case what I shall call its posterior angle of aperture 

 becomes considerable,) a truly achromatic construction is also 

 highly requisite. 



Fig. 2 represents a positive inverting eye-piece for astrono- 

 mical telescopes or for engyscopes used with micrometers ; it 

 is perfectly achromatic, and consists of two glasses, a and b, 

 with a certain interval between them, for the purpose of im- 

 provingthe oblique pencil. They maybe either achromatic and 

 aplanaticin themselves or by mutual correction. * I am sorry 

 to say that I have not been able to procure any combination 

 which gives so flat a field of view as I could wish consistent 

 with perfect distinctness. If it is thought worth while to ob- 

 tain a large field at the expence of centrical perspicuity, we 

 have only to reverse the position of the glasses, and that pur- 

 pose will be accomplished. 



I may observe, that if in these constructions the achromatic 

 glasses have their inner surfaces in contact, and are cemented 

 together, there will be no sensible loss of light beyond that 

 occasioned by two common lenses. 



I must now say somewhat about oblique pencils. They are 

 very much mistaken who suppose that they have not received 

 quite as much consideration as they deserve from those who 

 have interested themselves of late in the improvement of the 

 microscope. It is well known to every man practically acquaint- 

 ed with optics, and ought to be much better by those who know 

 the science theoretically, that in all constructions whatsoever 

 there is a perpetual system of gaining advantages in one di- 

 rection and losing them in another, just as in mechanics, — what 

 is gained in power is lost in time, &c. It is not permitted to 

 man, assisted by the most powerful analysis, to combine to- 

 gether every advantage he may desire. He must content him- 

 self with what is attainable. According to the laws of the re- 

 fraction and reflexion of light, I have ever found that all those 

 constructions which give a very large angle of aperture com- 

 bined with great distinctness, invariably produce what is called 



" Tlicir foci may be vurinl at pleasure, provided tliut tin- second or fichl- 

 glasi is not lesa than that of the first. 



