  Observations across the electromagnetic spectrum of radiative processes
involving interstellar dust -- emission, extinction, and scattering -- are used
to constrain the parameters of dust models and more directly to aid in
foreground removal of dust for extragalactic and cosmology observations. The
more complementary observations, the better. Here, we quantify the relationship
between scattered light and thermal emission from dust in a diffuse (cirrus)
intermediate latitude cloud, Spider, using data from the Dragonfly Telephoto
Array and the Herschel Space Observatory. A challenge for optical observations
of faint cirrus is accurate removal of a contaminating spatially varying sky
background. We present a technique to analyse two images of the same cirrus
field concurrently, correlating pixel values to capture the relationship and
simultaneously fitting the sky background as a complex non-correlating additive
component. For the Spider, we measure a $g-r$ color of 0.644$\pm 0.024$ and a
visible wavelength to 250 $\mu$m intensity ratio of $10^{-3} \times (0.855
\pm0.025)$ and $10^{-3} \times (1.55\pm0.08)$ for $g$ and $r$-band
respectively. We show how to use any dust model that matches the thermal dust
emission to predict an upper limit to the amount of scattered light. The actual
brightness of the cirrus will be fainter than this limit because of anisotropic
scattering by the dust combined with anisotropy of the incident interstellar
radiation field (ISRF). Using models of dust and the ISRF in the literature we
illustrate that the predicted brightness is indeed lower, though not as faint
as the observations indicate.
