Did Huygens understand light to be a transverse wave or a longitudinal wave? We have this source that claims Huygens "assumed light to be longitudinal", which contradicts this source which claims "Huygens believed that light was made up of waves vibrating up and down perpendicular to the direction of the wave propagation".
I must admit, neither source is entirely accurate or reliable, so what exactly was Huygens formulation regarding the nature of light?
 A: Possibly interesting quote
from the "Note by the translator" section (page ix)  of
"Treatise On Light" by Huygens, Christiaan
https://archive.org/details/treatiseonlight031310mbp/page/n10/mode/1up
(bolding mine)

The Treatise on Light of Huygens has, however, withstood the test of time: and
even now the exquisite skill with which he applied
his conception of the propagation
of waves of light to unravel
the intricacies of the phenomena of the double refraction
of crystals, and of the refraction of the atmosphere,
will excite the admiration of the student of Optics.
It is true that his wave theory was far from the complete
doctrine as subsequently developed by Thomas Young and Augustin
Fresnel, and belonged rather to geometrical
than to physical Optics. If Huygens had no conception
of transverse vibrations, of the principle of interference, or of the existence
of the ordered sequence
of waves in trains, he nevertheless
attained to a remarkably
clear understanding
of the principles of wave-propagation;
and his exposition of the subject marks an epoch
in the treatment of Optical problems.

