- Siemens's Continuous Tank of Dollond's invention of achromatic telescope objectives in 1 757, a demand first arose for optical glass, the industry was unable to furnish suitable material.
Some excellent instruments of the second type were subsequently made by Dollond's eldest son Peter, in which for the " convex glass within the tube " was substituted an achromatic object-glass, and outside that a divided negative achromatic combination of long focus.
Why, he might ask, should he not select the simple form of Dollond's first type ?
This construction would give all the advantage of the younger Dollond's object-glass micrometer, and more than its sharpness of definition, without liability to the systematic errors which may be due to want of homogeneity of the object-glass; for the lenses will not be turned with respect to each other, but, in measurement, will always have the same relation in position angle to the line joining the objects under observation.
Dollond's patent was not set aside, though the evidence with regard to the prior manufacture was accepted by Lord Mansfield, who tried the case, as having been satisfactorily proved.
It is very curious the conflicting evidence we have to reconcile, but I think the balance of evidence is in favour of there having been a prior invention of achromatic object-glasses before the date of Dollond's patent" (Astron.
It is, besides, impossible to read Dollond's memoir (Phil.