204 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



The application of the different powers of a telescope in general is of some 

 consequence; and in answer to those who may think I have strained or over- 

 charged mine, I must observe, that a single glance at the subsequent h Draconis, 

 n Coronas, and the star near y. Bootis, with a power of 460, showed them to me 

 as double stars; when, in 2 former reviews of the heavens, I had twice set them 

 down in my journal as single stars, where I used only the power of 222 and 227, 

 and in all probability should never have found them double, had I not looked 

 with a higher power. 



We are to remember, that it is much easier to see an object when it is pointed 

 out to us, than when it falls in our way unexpectedly, especially if of such a 

 nature as to require some attention to be seen at all ; but to say no more of other 

 advantages of high powers, it is evident, that in the research of the parallax of 

 the fixed stars they are absolutely necessary. If we would distinctly perceive 

 and measure or estimate extremely small quantities, such as a 10th of a second, 

 it appears, that when we use a power of 460, this 10th of a second will be no 

 more in appearance than 46", and even with a power of 1500 will be but 2' 30", 

 which is a quantity not much more than sufficient to judge well of objects and 

 distinguish them from each other, such as a circle from a square, triangle, or 

 polygon.* 



It has been observed, that objects become indistinct when the principal optic 

 pencil at the eye becomes less than the 40th or 50th part of an inch in diameter. 

 In the experiments that have been made on this subject it appears to me, that 

 the indistinctness which is ascribed to the smallness of the optical pencil may be 

 owing to very different causes: at least it will be easy to bring contrary experi- 

 ments of extremely small pencils, not at all affected by this inconvenience; for 

 instance, it is well known, that microscopes, consisting of a single lens or 

 globule, are remarkable for distinctness. We also know, that they have been 

 made so small as to magnify above 10,000 times.-f- From this we may infer 

 that their apertures, and consequently the diameters of the optic pencil at the 

 eye, could not exceed the 2500th part of an inch. I am therefore inclined to 

 believe, that we must look for distinctness in the perfection of the object-spe- 

 culum or object-glass of a telescope; and if we can make the first image in the 

 focus of a speculum almost as perfect as the real object, what should hinder our 

 magnifying but the want of light ? Now, if the object has light sufficient, as 

 the stars most undoubtedly have, I see no reason why we should limit the powers 

 of our instruments by any theory. Is it not best to have recourse to experiments 



* By a set of experiments, made in the year 1 774, I found, that I could discover or perceive a 

 bright object, such as white paper, against the sky-light, when it subtended an angle of 35" ; but 

 could only distinguish it to be a circle, and no other figure, when it appeared under an angle of 2' 24". 



+ See Padre Delia Torre's Method, &c. Scelta di Opusculi. 



