and on the Memoir of Eider. 245 



tainment of a more commodious method of observation. I cer- 

 tainly have seen many observers who prefer this form of the 

 refracting instrument, though I do not admire it myself. 



Remarks on Eiders Memoir. 



0. In page 6 of the first article of Eulers memoir, " Des 

 verres objectifs delivres de toute confusion," speaking of tri- 

 ple achromatic object-glasses, he says, " they might also serve 

 to improve miscroscopes by making them as small as possible, 

 but then you must reverse the order of the lenses, by turning 

 the third towards the object in its focus. To succeed better in 

 the construction of them, it will be advisable to inclose each 

 lens in a little cell, so that the distance between them may be a 

 little varied, in order to discover, by experiment, the best dis- 

 position of the glasses relative to each other, for it is almost im- 

 posible to execute in practice all the prescribed measures, but 

 a little change in their distance will be sufficient to remedy 

 this defect." 



When I consider this passage in conjunction with the curves 

 given by Euler, I am almost certain that he had no other 

 idea about the object-glass of an engyscope, but that it zoas 

 to be that of a very small telescoiye of a larger angle of aper- 

 ture them usual, reversed or inverted. The same notion I 

 have found to prevail among other mathematicians, with whom 

 I have conversed on the subject; and I must confess that in 

 theory the doctrine seems plausible enough, for when a tele- 

 scope is made to act upon a near object, say forty or fifty feet 

 distant, (and be it observed that opticians like to try them 

 best in this way, as they can be more certain of their quality 

 than when made to perform with more parallel rays,) there is 

 not a greater disproportion between the length of these con- 

 jugate foci than takes place in an engyscope, therefore the 

 rule ought, at all events, to be good enough for practical pur- 

 poses ; nevertheless, I am convinced myself that it is far too 

 rude and coarse; no optician could, I am sure, make a triple 

 object i^lass for an engyscope by it to please the most ordi- 

 nary customer, though it might do well enough for a mere 

 magnifier. 



