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The Herschel observations of nearby molecular clouds find ubiquitous filamentary morphology of dust emission projected on the plane of sky. Although the ubiquity suggests that the filamentary morphology of projected emission is the result of filamentary structures in the 3D space, the spatial structure in the line of sight dimension of a single filament is rarely examined. While a direct measurement is impossible, {\bf the line of sight ``thickness'' can be deduced from the dependence of molecular line emission on the volume density} and {\bf the dependence of the Spectral Correlation Function on the spatial scales of self-similarity}. Here \emph{we propose to observe the N_2H^+ (3-2) and the ^{13}CO (2-1)/C^{18}O (2-1) molecular line emission in the filament FN1 in the Serpens Main molecular cloud}, which will allow us to test and compare these two methods in measuring the line of sight ``thickness.''  \subsection{The 1st method: line of sight ``thickness'' measured by cyano-molecules}  Molecular transitions of cyano-molecules are sensitive to the local volume density. The ratio of emission from a higher transition to that from a lower transition is characterized by a sharp transition as a function of volume density (Fig. ?; 1;  Green \& Chapman 1978, Wernli et al. 2007). By measuring multiple transition of cyanoacetylene (HC_3N), Avery et al. (1982) and Schloerb et al. (1983) were able to derive a volume density for the TMC-1 region, and thus the line of sight ``thickness'' of the region by comparing to the column density measurements. Li et al. (2012) applied the same method to the Taurus B213 filament (Li et al. 2012) with HC_3N (4-3) and (10-9) transitions. The result in B213 successfully re-confirms that B213 is a cylindrical filament (Hacar et al. 2013) and has a ``thickness'' of 0.12 pc in the line of sight direction. This result also conforms with the width in the plane of sky of \~ 0.1 pc, as derived from the density profile across the filament (Pelmeirim et al. 2013). \subsection{The 2nd method: line of sight ``thickness'' measured by the Spectral Correlation Function}  The Spectral Correlation Function (SPF) measures the degree of similarity between two spectra, and is proposed to be applied on analysis of spectral maps (Rosolowsky et al. 1999). Padoan et al. (2001a) further conclude that there is a dependence of the SPF on the ``spatial lag'' between the two spectra that the SPF takes into account. This dependence of the SPF on the spatial lag shows a power-law relation, and the spatial scales where this power-law relation exists characterize the spatial scales of self-similarity of turbulence (which is assumed to dominates the spectra; Fig. ?). By computing the self-similar scales characterized by the SPF of HI spectra and assuming that the self-similarity of turbulence is confined to the shortest dimension in the 3D space, Padoan et al. (2001b) measured the ``depth'' (scale height) to be \~ 180 pc in the line of sight direction of LMC, which has a face-on disk structure and thus the shortest dimension along the line of sight.