A Dioptrick Problem, Why four Con^ex-glaffes iu a- 
Telefcope^ (hew GbjeSls EreSL by William Moli- 
neux e?/" Dublin Efq. R. S. Soc. 
IN the Jourml des Scavdns for Munday the i -Jth. of Sep-^ 
tember 1685. pag. ^66. Amfl. Edition, we find this paf^ 
sage. As Perfpe^ives of one Convex-glafs make Objects appear 
Upright y which thoje of two Convex-glajfes invert ^ and again 
thofe of three re^ify ; fo it fbould feem that thofe of fetir ought 
to invert \ Jnd yet Experience jhews us that Objects appear up-^ 
right through thife glajfes. The Singularity of this Phsenomenon 
obliges all Skil^d in Dioptricks to inquire tbe reafon thereof^ but 
hitherto they have found none. Mr. V^tgis^ who applies himf elf 
particularly to this part of Natural Philofophyy heleives that he 
has hit upon the Reafouy and makes us hope that he will fuddenly 
Pdlifhit. i 
Thus far tht 'Journal^Vit it does not tell^ t^ whofe remark 
this is, though ! am apt to beleive 'twas written by Mr. 
Regis X-iim&n^ to the Publifher of the 5^<?/^rW. 
To me this Phenomenon appears very eafily explieablc, 
from the confideration of placing GlalTes in aTube, yt^hich 
is thus ; after the Obje^^glafsy the Eye-glafs is placed fo much 
diftant ( towards the Eye ) from the F^?«/j of the Objeff- 
glafs as is the Focus of the Eye-glafs ; then the middle Eye-glafs 
is placed lb much diftant from the Focus ot the firft Eye-glafs^ 
a-s is thei^(?^^/j of thismiddle E^^-^/^/j^ neareft Eyc^ 
glafs h placed ib much diftant frcm the Focus of this middle 
Eye-glafs ^ as is the Focus of this neareft £^^»-^//1/} ; and the 
Eye looking through them all is placed in the Focus of this 
\*nQ^t^^^ye'gkfs. 
I fay therefore firft, that one fingle Convex-glafs, cannot 
properly be faid by it felf to fhew Objefits ereft or reverie, 
but in refpeft of placeing of the Eye that looks through it. 
•For if the Eye that looks through fuch a fingle Convex-glals 
X be 
