200 MR. DOLLOND ON A CONCAVE ACHROMATIC LENS. 



Thus it will be seen that we have the advantage of using longer eye-glasses with an 

 extension of power, whereby the wires or spiderwebs of the micrometer are not 

 increased in diameter, a very essential advantage when observing minute double 

 stars ; nor is the eye of the observer so much distressed as when the magnifying power 

 is obtained by shortening the focal lengths of the eye-glasses. 



The advantages of this improvement having been shown by the foregoing intro- 

 duction, I will now proceed to give an account of the causes which led to its being 

 applied to the micrometer, and the result of its application. 



The Rev. W. R. Dawes of Ormskirk, a gentleman pursuing practical astronomy 

 with great zeal and perseverance, and to whom the public are already much indebted 

 for several valuable communications, being desirous of carrying his measurements, &c., 

 of the double and revolving stars, to a greater extent than the powers of his micro- 

 meter then allowed, applied to me to construct for him an arrangement of eye-glasses 

 that would increase the magnifying power of his telescope without increasing the ap- 

 parent diameter of the spiderwebs in his micrometer, or interfering with the mode of 

 illumination. Several combinations were tried without success, when it occurred to 

 me that the achromatic concave lens, which had been decided by Mr. Barlow to be 

 of no use for his purpose, might accomplish what was required. 



The result I will now state from a letter I soon after received from Mr. Dawes, to 

 whose micrometer this improvement had been applied. 



" Ormskirk, March 14, 1833. 



" My dear Sir, — You will doubtless be surprised at not receiving from me any 

 account of the performance of your scheme for the improvement of the achromatic 

 telescope. 



" My general opinion of your improvement is, that it is, for the purpose it is de- 

 signed to answer, as useful as it is elegant. 



^^By a careful determination of the value of the micrometer divisions, I find the 

 magnifying power of any eye-tube is increased in the proportion of 2*1068 to 1 : each 

 part originally = 0""555922 is now = 0"'263867. To obtain the magnifying powers 

 of the eye-tubes, I content myself with multiplying the original powers by 2*1. But 

 I will detail a few particulars noted in my journal on the subject. I have ttms set 

 down the advantages of the additional lens. 



" 1st. The micrometer threads are only half the thickness, with the same magni- 

 fying power on the object ; small stars are therefore neither obliterated nor distorted. 



" 2nd. The parallel threads are both very nearly in focus with any power up to 

 600 ; before, only up to 285 (the same eye-piece). 



" 3rd. The value of the micrometer divisions is less than one half its former amount, 

 permitting a proportionally fine motion in measuring the distances of delicate objects. 



" 4th. A much greater extent of the field being flat, and the threads distinct 

 further from the centre ; of great importance in accurately determining the zero of 

 position by the passage of a star along the thread. 



