236 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



sure its picture ab, as in fig. 18. The power of one lens therefore being known, 

 that of the rest was also found by comparing the measures of the projected 

 images. 



It may not be amiss to mention some of the advantages and inconveniencies 

 attending each of these methods. When we take the focus of an eye-lens, 

 which the first method requires, we are liable to a pretty considerable uncertainty, 

 and in very small lenses it is not to be done at all. Also, in calculating the 

 power by that focus, no account is made of the aberration which takes place in 

 all specula and lenses, and increases the image, so that we rather find out how 

 much the telescope should magnify, than how much it really does magnify; but 

 in determining the power by an experiment we avoid these difficulties. On the 

 other hand, when the power is very great, the latter method becomes inconve- 

 nient, both on account of want of light in the object, and a very considerable 

 aberration which takes place, and makes the picture too indistinct to be very ac- 

 curate in the measure, and of course larger than it ought to be ; and this will 

 account for the excess in the measures of my 2 largest powers. However, when 

 I employed 6450 on the diameter of a Lyrae, I incline to think the method I had 

 used when I determined that power, ought to be preferred, because my lamp- 

 micrometer gives the measure of an object as it appears in the telescope, and 

 therefore this aberration is included, and should be taken into consideration. 



To prevent any mistakes, I wish to mention again, that I have all along pro- 

 ceeded experimentally in the use of my powers, and that I do not mean to say 

 I have used 6450, or 5786, on the planets, or even on double stars; every 

 power I have mentioned is to be understood as having been used just as it is 

 related; but further inferences ought not as yet to be drawn. For instance, my 

 observations on £ Bootis mention that I have viewed that star with 2010, or as 

 in the above table with 2175, extremely distinct; but on several other celestial 

 objects I have found this power of no service. Many plausible suggestions have 

 already occurred to account for these appearances; but I wait till further experi- 

 ments shall have furnished me with more materials to reason on. The use of 

 high powers is a new and untrodden path, and in this attempt variety of new 

 phenomena may be expected; I therefore wish not to be in a haste to make general 

 conclusions. I shall not fail to pursue this subject, and hope soon to be able to 

 attack the celestial bodies with a still stronger armament, which is now preparing. 



XIV. Continuation of the Experiments and Observations on the Specific Gravities 

 and Attractive Powers of various Saline Substances. By Rich. Kirwan, Esq., 

 F.R.S. p. 179- 

 Before entering (says Mr. K.) on a detail of the new experiments I have made 



in the prosecution of this subject, I must beg leave to rectify some mistakes I 

 have fallen into in my last paper. 



